Most answers have already went into 1 so for that I will just say:
The starting conditions for creating astronomical objects are messy (the particles in the gas clouds are not perfectly symmetrically distributed) and that messiness inevitably leads to rotation and revolution.
I mainly wanted to focus on 2:
Why doesn't the speed decrease ?
Because of Newton's first law: An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force.
Since gravity of sun tries to pull it, each time, it should be causing the earth to slow down
Imagine holding a string with a ball at the end. It doesn't matter that the ball is constantly being tugged by earth's gravity, it's going to stay still because the gravity is perfectly balanced by the tension of the string. The net forces involved aren't changing, therefor there can be no change in acceleration because F=MA (and thus no change in motion).
And that's what is going on with the Earth-Sun system. For simplicity sake, let's assume the orbit is circular. Then even though sun is constantly pulling, there is no NET change in the forces involved. For Earth's orbit to decay, by Newton's first law there must be a net change in the force on earth (otherwise the object in motion will stay in motion).
If I had a car with an engine
A car constantly needs to apply positive force (that is, force in the direction of the motion of travel) because it is constantly assaulted by both wind resistance and friction which is applying a negative force against the motion of the car. To maintain constant speed the forces must be balanced or the car will either slow down or speed up.
Now, for many orbits similar things are happening and that is why many orbits do decay. Objects in low earth orbit encounter friction from particles which slow them down and crash them back to earth. The moon and earth interact in non-trivial ways through tides and other things and those interactions result in substantial changes over time. Orbits around the moon decay because the moon's mass is not evenly distributed and thus the moon cannot be treated as a point particle by objects in orbit around it.
But for things like the sun and the planets those issues are completely negligible. Yes, the earth also encounters friction from particles while in orbit but the forces involved are for all intents and purposes 0. The sun is far enough away that it can be treated as a point-like mass and there are no significant tidal forces. So the Earth continues at the same speed because there's nothing there to slow it down.
On a final note, there is something that can decay orbits of large massive objects: gravitational waves. This is why massive black holes in orbit around each other eventually merge -- their orbits lose energy in the form of the waves and that loss of energy translates to them getting closer and closer together. But like the other things in the Earth-Sun system, the net force from gravitational waves on the scale of objects the size of the sun or earth is so small it is essentially unmeasurable.