P90X is a fine workout for someone who doesn't know much about training and just needs something to follow. For someone who doesn't normally train they will lose fat and gain strength. Eventually though, once you stopped gaining, it would be best to get something a little more customized to your needs.
For now though, as a beginner, give it a shot! Don't give up as it'll be really hard at first, but stick with it. Don't forget to keep an eye on your diet too, the exercise is only half the battle!