This is a very common question, yet very hard to answer if you prefer a clear, concise, uncontroversial answer that applies to all situations. So I'm not going to do that.
Instead, I'm going to describe your main options, and let you choose. Be aware that you'll make the choice while still not knowing much about optics. So, in a sense, it will be just the beginning of a learning journey, and you'll have to keep learning and trying things out. Hope you like that sort of thing, because it's kind of mandatory in this hobby.
Also, there are two kinds of people in this field: those who enjoy using the instruments, and those who enjoy making the instruments. I'm 25% of the former, 75% of the latter kind (I make telescopes and mirrors, and I do some optics design). So that's the perspective you'll get.
First off, a few words about the objects you'll observe. There are two kinds of objects:
HIGH RESOLUTION, HIGH BRIGHTNESS
These are the planets, the Moon, many double stars, and the like. These are typically pretty bright, so the amount of light you capture is not essential. But these objects show high-resolution detail, so the resolving power of your scope is crucial.
You get more resolving power if the aperture (the diameter) of the scope is bigger. But there's a problem - seeing (the inverse of turbulence).
When air is turbulent, it will reduce the resolving power of your instrument. If your scope is 200 mm (8") or bigger, this will happen A LOT. If your instrument is 100 mm (4") or smaller, it's not so noticeable because you don't have that much resolving power to begin with.
There are ways to forecast seeing, because it depends on weather:
So, for high resolution targets, the point of having a large instrument (like a huge dobsonian) is because once in a while seeing is so good (turbulence is so low) that you can actually use all its huge power - but most of the time you'll be using it at much less than its theoretical power, as if it's a 4" instrument or whatever the seeing is at the moment.
These objects are not affected by light pollution. You could observe the Moon from a brightly illumined parking lot in the middle of the city. You don't need to be in deep darkness while observing them - in fact, it's beneficial if you're not adapted to very deep darkness.
These are most of the nebulae, star clusters, and galaxies. These are distant, faint objects. Their light is very faint, so it's essential you capture as much of it as possible. Again, bigger aperture is better.
These are the kind of objects that huge dobsonians (half-meter aperture, or more) are made for. In a very large scope, there's galaxies everywhere.
But again, there's a catch. These objects are affected by light pollution.
You need to take your scope far and away from any city, town and industrial complex (at least a 1 hour trip at highway speed). This is pretty hard to do in Europe; in the US, population density is low and this is somewhat easier. Anyway, just stay away from city lights and the view will improve.
A few of these objects will be visible from the city: the Andromeda galaxy, the M13 cluster, the Orion nebula, etc. But many of them look much, much better away from light polution. In any case, you should be able to observe the whole Messier catalogue (over 100 objects) from the city - but out in the boondocks these things look better.
It's important that your eyes are adapted to very deep darkness when observing these things, so stay away from any source of light. Even the screen of your phone is far too bright. Street lights 500 meters away are too bright. And turn off that flashlight and keep it off. One second of exposure to a bright light destroys a dark adaptation that took 20 minutes to build.
Since these objects are so faint, the eye doesn't see a whole lot of high-resolution detail, so they are not affected by seeing (as far as the human eye is concerned).
Okay, now here are the main types of instruments that you're most likely to encounter:
NEWTONIAN REFLECTORS IN GENERAL (DOBSONIANS IN PARTICULAR)
Many people will say "just get an 8 inch dob as your first scope", and that's usually correct. This design gives the biggest aperture per price. Biggest bang for the buck. It scales up easily to very, very large sizes. It's the ideal instrument for observing the "faint fuzzies" (nebulae, galaxies). It's great for learning, because you can easily observe its guts. When seeing is good, a large dob will open up the sky for you - you can crank up the magnification a lot (depending on aperture).
On the flip side, due to the low price, you will see many dobs out there that are shoddily made.
You need to remember two things with a dob: collimation, and thermal management.
Collimation means keeping all the optics aligned. The mirrors will move a little, due to the way a dob is built, and they'll lose collimation. So don't forget to re-adjust it once in a while. In time, you'll get so good at it, you'll give it a two-minute quick adjustment every time you observe, and it'll be good to go; that's what I do.
This is why collimation is important:
This is a good introduction to newtonian collimation (which is a vast topic, you could write a whole book on it):
Thermal management means keeping the scope at the same temperature as the ambient air. If you don't do this, internal convection will destroy the image. This is why:
Small dobs (up to 6...8"), just take it outside at least 1 hour before you observe - or better yet, 2 hours. Let it "breathe". Dobs 10" or larger, you MUST use a fan on the primary mirror to force cooling.
These instruments are very convenient and very low maintenance. Take it out of the box, and it's good to go. It's collimated from factory, and it doesn't care about thermal issues. A refractor makes a great "travelscope", and a great grab-and-go scope.
On the flip side, they are the most expensive, in terms of how much aperture you get for the money you spend. And they are typically made in small apertures; most commercial refractors are not bigger than 6" or so; those that are bigger are extremely expensive. The small aperture limits the maximum performance of these instruments.
There is this myth out there that refractors are somehow magically better for planets (and other high-resolution targets), whereas dobs, even very large ones, are not so great. This is false; load up the designs in Zemax, do a basic performance analysis, and you'll see that aperture is the dominant factor, with everything else being secondary. But there are several reasons why this myth is around, and it's instructive to see what these reasons are:
- refractors perform at 100% of their potential no matter what, right out of the box. Dobs require maintenance and fiddling to perform at 100%.
- many dob owners simply don't do the required maintenance - they don't collimate their scope, and don't give it time to reach thermal equilibrium. Then of course it will perform miserably on planets. There are 12" dobs out there that perform worse than 4" refractors because of how out of collimation they are and how hot the mirror is.
- many refractors out there are quite well made (they ought to be, given the higher price you're paying), whereas many dobs are cheaply made in a sweatshop. So you're not comparing apples to apples.
Bottom line: don't buy a refractor because you "want to watch planets". But do buy a refractor if simplicity of use and the low maintenance are very important to you (and you don't care much about price).
A decent refractor on an excellent mount can do pretty good astrophotography. More on that below.
CATADIOPTRICS (CASSEGRAIN, MAKSUTOV, DALL-KIRKHAM, RITCHIE-CHRETIEN)
Broadly speaking, these are for astrophotography (not always, but usually). But can you do visual astronomy with them? Sure. It's just that it's easier to optimize them for photo, with this design.
Usually, they come installed on a motorized mount. All but the very cheapest ones have a computer on-board; you just punch the name of the object into the remote control, and the scope will turn around automatically to face the object. Sounds cool, right?
Well, I am going to say something a little controversial here: this is not a good place to start.
First off, astrophotography has an extremely steep learning curve. It's very easy to take a little blurry photo of Jupiter that will impress nobody. It's very hard to take a great, high resolution photo like these:
The skills and knowledge required to take that kind of photos are more easily acquired by doing purely visual astronomy, on a dob or whatever, for a couple years or more. Also, the equipment required for that sort of stunt costs many thousands of dollars - your cheap little Cassegrain cannot do it.
So why buy a small Cass? Well, if you have an overwhelming interest in astrophoto, and are prepared to deal with the difficult learning, then go ahead. But realize that the first images are not going to be very impressive, and the motorized mount will not teach you some valuable skills that you would acquire more easily pushing a dob around and keeping your eye plugged into the ocular for a while. Also, money spent on the motorized mount is money NOT spent on aperture, and the aperture is so important for overall performance.
For astrophotography, the mount is of huge importance; the scope per se is relatively less important (I'm generalizing). A so-so scope on a great mount is much better than the other way around. But a great GEM mount can be very expensive.
Well, that's it. Do some more research on your own, then make a decision. Don't forget that learning only begins with the moment you acquire the scope.
Above all, have fun, and keep looking up.
Clear skies to you!