You are definitely doing the right thing by asking for advice. Keep that up. Learn all that you can. Ask the guys here, ask the guys in the gym, read magazines, etc.
If I were you, I would rewrite your question and tell the guys here what your goal is: more muscle? slim down? Be a fast runner? Have more endurance? Because how you train depends on your goal, it would be helpful for you to post a picture here of the kind of body that you are training for. That will help the guys here give you advice.
I agree with the poster above me. The first thing you'll want to do is decide what your goals are. If you are looking to get big muscles, you'll want to focus more on weights (higher amount of weight but with a lower number of repetitions) and raising your quality calories. You may not want to do as much cardio because that will burn a lot of calories and keep you smaller. If you are looking to get tone without getting bigger, then you'll still want to focus on weights, but a little differently (lower amount of weight, but with a higher number of repetitions). You'll want to do cardio also to stay trim. If you are are working on a performance goal (faster, stronger, more endurance, etc.) then you'll want to train for that specifically.
You said you were going to do p90x. From what I know of the program, that is going to help you get a leaner toned body. If you are already thin and you want to get "big", that probably isn't the best program because there is a high amount of cardio in that program, so it will be hard for your body to get big. Also, a lot of the exercises use lower weights with higher reps. That is great for toning up, not for bulking up. A friend of mine went through the p90x program. He started off with about 15 pounds of extra weight. When he was done, the weight was gone and he definitely looked a lot better because his muscles hot gotten a little bigger and were toned. The program had a very positive effect on his self confidence and overall well being.