Hello!
Depends on what type of aircraft we are talking about here. I suppose you are referring to a large aircraft fuel tanks. B707 and DC-8 tanks are tricky and uncomfortable to work on (not that fuel tanks are a great place to work on). I've worked on different types, and just make sure you follow the maintenance manual procedures while accomplishing this kind of work.
Most tanks must me emptied and well ventilated for at least 48-72 hours before you "jump in". You must have a suitable respirator to keep your lungs away from the fumes. Keep your skin protected. Very important is the use of a partner outside of the tank to keep you "awake" and pull you out if you have any emergency.
Regards,
Airliner777