The good news is that there are only about four or five things that could be wrong, and only one of them (bad starter) will cost very much.
There are some simple diagnostic steps.
1. Is the starter actually spinning but not engaging the flywheel (you will have to have your head down really close to the starter while someone hits the start button as it is amazing how quiet a starter is if it's not actually turning the motor over. If it is running, it's the bendix problem.
2. If it is not turning, the next thing to do is jump the solenoid by connecting the two posts together on the starter that the solenoid is connected to.
If it turns over, you have a bad solenoid or a bad starter switch.
3. If it doesn't turn over then, you have a bad starter.
That's a very rudimentary instruction, and not every possible thing that could be wrong, but a good book (like Nigel Calder's) is a good place to learn to do stuff.
Either that or have driven a lot of POS cars and trucks growing up that you had to fix all of the time.