By the sounds of it... your selector has broken. This is the part you will be looking for...
http://www.evike.com/product_info.php?products_id=29896 Luckily, its very easy to get to it.
1) Make sure the chamber is clear, and mag is removed.
2) Pull your trigger guard down, and then forward towards the muzzle.
3) Remove trigger guard assembly.
4) Lift rear end of the receiver.
Part linked should be "wrapped" around the back of the receiver.
This is all from memory, from when I had my m14 AEG. Others should be able to chime in, in the event I forgot something.
Edit: very good picture guide...
http://www.airsoft-barracks.com/forums/ ... own-guide/
character density, n.: The number of very weird people in the office.