Unfortunately, someone asks the same thing just about every other day in here. You'll have a hard time getting any answers unless you do some research and ask specific questions. All the answers you need are in this forum, so I'd recommend you start looking through the threads or do some searching.
As far as your question goes, just bondo is a bad idea. If you want it to last, you'll need something more structural like ABS goo or epoxy or even fiberglass for larger openings. A good place to start off your research would be here.
EDIT: I see you're not a newb to the site...shame shame