If you are most people, don't listen to
>>4 because you have little access to a trampoline and are unlikely to ever do anything.
Also, don't have someone spot you, you'll look like a pussy fag and everyone will hate you.
Just man up. If you are anything better than the worst possible person at backflips you'll just land with a belly-flop. Still, if you are really wussy, stick with the frontflip until you get really good at that, because there is no way you'll break your neck doing that unless you're the type of person who shouldn't be doing anything physical other than walking. At least by getting the front flip down first, people will still think you're cool. Unless they ask you to do a backflip...
Also, do some ab workouts, to get a flip you have to curl yourself up midair, and that will require at least a little muscle. Plus, you can work your abs anywhere, unlike trampoline work -.-