If I have a good idea for a skin, and a draw it what are the chances Riot will use my skin?

Depends on how popular the idea becomes. I know for certain the suprise party Fiddle was a fan idea before it became a skin. I think the same is true for pool party Zac and a few others.

If nothing else, making the skin and showing it off will be good display of your skill and get you some attention

You will get no money, ever. Let me put it to you this way: if you drew a picture of a car and showed it to a car company, you would get no money when they built said car. See here's the fun thing, you can create fan art of League of Legends characters, and Riot cannot sell it because you drew the picture. But, Riot can insist that you take the drawing down and not be allowed to sell it because they own the licensing rights to the characters.

You won't get paid, and it probably won't happen unless you're employed officially and get the project going yourself