i got a rust cnear my rear wheel on the left side and the paint came off today! i'm just wondering how i would go about fixing this before it spreads like crazy

take a look at the pics for the rust spot !

 

either sandpaper or a fiberglass pen from amazon

 

That isn't very bad. I would sand it with sand paper, put some primer on it and paint over it with touch up paint.

 

First, wash the area with water to remove any debris, wax, residue. Sand inside the crack (use a small flat head screw driver inside the smaller cracks if you have too, just scrape at the rust and don't go too deep) to remove any lose paint and surface rust, then take epoxy primer and fill the crack by dabbing it in with a brush. After that dries, take your touch up paint and dab in the crack over top of the primer, don't brush it, just dab it in gently. Repeat for 2-3+ coats or until paint is even with existing paint and you're done.

 

Chances are its under the crack so all you can do is put a band aid on it. Sand it a little and touch it up with a brush.