How big is the hole?
If it's small, it's as easy as: mix, apply, dry and sand.
(for holes smaller than a finger nail)
For larger holes you will want to use a fibreglass patch over it with bondo.
But before you even do either, you must strip all paint down to bare metal, either by sanding, or using a paint stripper. Then clean the area thoroughly before applying filler (or "bondo" as we like to call it).