I've found the best way to get into, and stick with, a fitness program is to tie it to an interest, e.g. instead of going for a run and lifting weights, take up an exersize-styled hobby like boxing, pilates, TaeKwonDo, yoga, or whatever you think might hold your interest. The best interests normally are ones where you can develop in a comparable way, and you can practice with people, but you can also practice alone.
Football is really popular in the UK, and while I personally hate it, it is a good way to stay fit, and if you like it, have fun at the same time.
Other than that, have a good idea what kind of fitness you're aiming for (I.e. fighting fitness, endurance fitness, sprinting fitness, etc) - to get to a high level in any of them requires different training to each of the others.
The last point is to do a bit of research. Building muscle, for one thing, has its own science to it, that most people hardly know about. If you research what you want to accomplish beforehand, you'll get much better results.