i don't understand how people solo?

Like people have said above, so much of that is based on scales. Look to the scales to find the "good notes" in a situation then start looking at the melodies of the song/part. Getting a good handle on those two things will get you going on the basics, and honestly carry you a very long way. Once you have a handle on that you can start looking to the styles of your favorite guitarists. Noticing how they expand on those ideas (or go "off book" entirely). Of course there is also a whole lot of practice involved.

Learn solos from songs that you like, otherwise doesnt matter how many scales you practice you probably wont be able to come up with any good ideas, then learn some scales and go from there :)

1. Practice

2. Listening, learning, and adapting what you've listened to and learned into something of your own

3.Practice

4.Learning scales, Modes, and basic theory helps tremendously

5.Having a good ear

Scales and timing are the key, once you dominate scales and practice a lot to play them faster (along with techniques like bending, slides, tapping, hammering, etc.) you must try to play them accord to the compass of the song and make your timing correct. And yes, is dfficult as hell, but with lots of practice and never giving up you'll make it.

I have never heard random solos that sounded good. why not rehearse a solo first and then present it to the public later?

practice, self discipline, motivation, passion