How does orbiting work exactly? [duplicate]

Question

First of all I'm not a physicist or anything and I don't know much about physics, so I'm sorry if this doesn't make sense at all :) And if possible please try to explain it with words instead of complicated formulas, because I probably won't understand them.

Let's take earth and sun for example. From this picture I understand that earth is being pulled by sun constantly but I don't understand where the opposing force is coming from. How can earth still keep opposing sun's gravity for millions of years. Where is the force coming from? Shouldn't earth be slowing down and eventually collapse into sun? If no then does that mean that there the opposing force is being constantly applied. Or the earth had some initial momentum and it never loses that momentum for some reason? But that doesn't seem intuitive, because if you throw something on earth it falls down because gravity is stronger than that momentum. But in space if earth is being constantly pulled by sun and earth is opposing that pull constantly, shouldn't earth need to use energy to oppose the pull and eventually lose all of it's momentum? I've heard about the theory that earth is actually constantly falling into sun. But I can't understand that one, because I don't see why is earth evolving around sun in that theory, it seems like it should just follow sun in a straight line if it's constantly falling into it. If it's evolving then why doesn't sun crash into earth when earth is in front of it? To sum it up here are my questions:

1. How did earth get it's initial momentum?
2. Is that momentum ever slowing down? If not, why?
3. Is there a constant force that gives earth it's momentum, or it's just the initial momentum that never slows down?
4. If the earth is constantly falling into sun, how can it also revolve around it? Shouldn't it go in a straight line after sun?

Answer 1

How did earth get it's initial momentum?

From the gas and dust cloud that condensed into the sun and the objects that currently orbit it. There is a good article on this process Formation of the Solar System

Is that momentum ever slowing down? If not, why?

In practical terms, no, this angular momentum of the Solar System will continue until the Sun eventually becomes a red giant star: Evolution of the Solar System. Angular momentum is conserved, because there is nothing present in the Solar System to reduce it, such as another gas cloud entering the Solar system.

Is there a constant force that gives earth it's momentum, or it's just the initial momentum that never slows down?

This is answered above, a constant force applied to the Earth would speed it up in its orbit, until it travels away from the Sun, which is not occuring.

If the earth is constantly falling into sun, how can it also evolve around it? Shouldn't it go in a straight line after sun?

I guess you mean revolve, not evolve. The rotation of the Earth around the sun is a balance of two forces, one force pulling it towards the Sun, the other force keeping it moving in a straight line. This is illustrated in the article: Circular Rotation, although as noted in the comments, the planets move in elliptical paths rather than circular orbits.