I'm a noob when it comes to trail maintenance and my rule of thumb is this: I'll do stuff to put the trail back to its "original condition" whatever that is. That's probably a good starting point for someone that's just trying to help out. Fix bridges and stuff that get broken, clear small branches (deraileur hangar breakers) that fall on the trail, pick up trash, block illegal short cuts. Stuff like that. That's what I do. It's pretty satisfying because you WILL notice the quality of the trail improving as you maintain it, especially if it's poorly maintained to start with.

I'd say away from major changes like moving logs, moving rocks, cutting roots until you know what you're doing.