This is a very broad topic. You can literally write entire books on it. I'll stick a short answer here for future reference.
You have two main options: refractor or reflector.
Refractor is easier. You'll need a relatively large lens with a long focal length - this will be your primary optics, at the top of your telescope. The length of your scope will be dictated by the focal length of the primary lens.
You can measure the focal length by going outside when the Sun is up and trying to focus the Sun to a tiny pinpoint image - the distance then between lens and image is the focal length.
As an ocular or eyepiece you could use a smaller lens with a short focal length. Much better, you could use an eyepiece from an old microscope or binoculars.
The total distance between the objective (primary lens) and ocular (eyepiece) is the sum of their focal lengths. If the primary lens has focal length F1, and the eyepiece has focal length F2, then when looking at infinity the distance between them is F1 + F2. This will help you calculate the total length of the tube.
However, if the ocular (eyepiece) is a divergent lens (the kind of lens that makes things seem smaller) then the total distance is F1 - F2. By the way, it's hard to measure the focal length of a divergent lens, because it doesn't make an image you can project.
The magnification of the instrument will be:
M = F1 / F2
Do not worry too much about magnification. Make something that works first. More magnification is not always better. In the beginning, aim for something in the 10x ... 50x range. Refine it later.
Make sure the tube is rigid. A plastic pipe like the ones used in water installations should work well.
Make sure all lenses are centered and aligned with precision. You don't want your lenses to be leaning in any direction. Their main axis should coincide with the axis of the scope. This is important for performance.
If you get too many color fringes (little rainbows) around objects, try and reduce the diameter of the primary lens by placing a paper ring in front of it - this makes the image a bit darker but should diminish the pesky rainbows somewhat.
Mount the eyepiece on a smaller tube that's sliding into the main tube. Cut the main tube a little shorter than F1 + F2. This will allow you to move the eyepiece back and forth until the image is in good focus.
If you find a shop that makes prescription eyeglasses (for people with eye or vision problems), they have so-called "blank" lenses that they use to cut the glasses from. See if you can buy one of those for your primary optics. A focal length of about 1 meter should be fine (opticians call it a 1 dioptre lens, which you can find in pretty weak prescription glasses).
Sometimes you can pull a good lens out of a broken projector or something, but make sure the diameter is big enough (50 mm or bigger should be fine) and the focal length is big enough (I would say anywhere between 0.5 and 2 meters - that's between 2 and 0.5 dioptres - but keep it close to 1 m if possible).
The first telescope I made, many years ago, was very much like this. A blank lens from a prescription glasses maker, a plastic pipe, an eyepiece from a toy microscope. It worked reasonably well, I could see with it craters and mountains on the Moon.
Making a reflector is much harder, but the performance of the instrument is higher. Read the Stellafane guide, it's very comprehensive:
What's described in there is a dobsonian telescope. That's a newtonian reflector on a simple alt-azimuth mount.