Well if you go through your drills and shut down the engine. Then check for fire if the fire is still there ie, T handle illuminated then continue with Emergency Shut off valve and Pull that dam handle. Assuming your talking Otter ?
Land ASAP,, if it isnt out, you may have to ditch! In a light twin you have around 5 minutes before structural damage affects your aframe. Ie. flight characteristcs. SO GET IT ON THE GROUND ASAP IS ALL I CAN SAY.
I used to fly Old King Airs that had thermal loops and the Sun at the rowrong time of day gave Fire WARNINGS on the announciator panel.
Regards
Sheep