Loose rear deraillure?
 

Hey guys, my friend and I were riding our bikes when he tried to change gears. It seems like his rear gear "deraillure?" (not sure) popped out. here is a picture. any ideas on how to fix it? Thanks

http://img136.imageshack.us/img136/3056/dsc00039xu5.jpg

I cant see anything wrong from the picture, but the shift cable may not be firmly attached under the derailler. Go to this site for instructions on adjustment http://www.parktool.com/repair/byregion.asp?catid=53

sorry for being a noob but which repair how to should I look at

What do you mean by "popped out"? Everything looks to be attached correctly. Or did you mean that the bike shifted on it's own? A mis-adjusted cable-tension can let the derailleur ride in between gears, then adding pressure from the pedals can cause the chain to skip to the next gear. Rusty chain also doesn't help as it may have stiff spots.

Check out the Park Tool self-help repair site for tips and procedures on fixing bikes.

1) lube that chain
2) ambiguous problem descriptions don't help us solve your problem.

I think the first one listed "Rear Derailler Adjustment" is the one you need, but there will be no harm in looking at all the others as they will help you understand how the system works.