If you're going to ride it's best to know some basic maintenance; I for one don't want to bother someone on their ride just because I didn't bother to learn something simple I can do myself.

1) Changing a flat - If you can't do this don't ride a bike.
2) Tightening Bolts - This is as basic as it gets and if you can't turn a screw then you have bigger problems.
3) Simple Brake/Derailleur Adjustments - Similar concept to turning a screw, just make sure you read up on the topic so you don't over do it.