FIRE!

One of the most dangerous things in aviation is an in-flight fire.  So my question is what are you going to do and how are you going to do it?  If you do a little research into the topic you will discover that you don't have much time to handle the situation.  The State of Wisconsin lost a light twin several years ago.  The airplane was consumed by fire in less than a couple minutes.  The AOPA just relaunched a video about a C-172RG with an inflight fire.  The CFI and the Student where burned severely even after they go the airplane on the ground in less than 3 or 4 minutes.

The best bet is to get the aircraft on the ground as quickly and as safely as possible.  If you look in the POH it may say to fight the fire and do an emergency descent.  If there isn't a procedure for the emergency descent, review the procedure in the FAA's flight training handbook.  

Fire is not something to mess around with.  Have a plan and get the airplane on the ground as quickly as possible.  Good Luck!