I would of explained warps too, but Jelze did so, here is some useless basics.

-Dont make your level to frustrating, races should be fast, not barely possible. If it will get to annoying, the player will hit ESC and quit.
-Just because its a race, doesnt mean it has to have horrible eyecandy. Any lvls, even short ones, need good eyecandy.
-Work on gameplay as well, if your not sure, you should have others test your lvl and point out a few bugs.
-Dont add pits where your player can get stuck
- Do not make it very long, remember, a race has at least 2 or 3 laps, if you make your levels as long as cooperative lvls should be, it will get ver very long and boring.
-Add some puzzles, make the player look smart, remember, there can be more than one passage way.

Im not a race expert, so thats practically most of the advice I can give you.
Only upload it if its good!!!