Can I just start off mentioning that having four of the first five posts in a thread in the Beginners forum just to bust the OPs balls, is kinda screwed up, especially when the OP doesn't seem like any kind of over-the-top troll job.
Believe me, I get it. It can be tiresome seeing the same questions over and over. But mocking the OP, who's just a kid, isn't really helping anyone. [/soapbox]
For training, start off with a basic bodyweight plan:
Mon., Wed., and Fri.
Squat 2x15 (No weight, keep both feet flat on the floor.)
Push-up 2x15 (On your toes, go until the chest almost touches the floor.)
Lunge 2x15 (Alternate legs, 1 rep left/1 rep right.)
Neutral-grip pull-up 2x15 (assisted if necessary. Could be substituted with inverted rows.)
Plank 2x15-count (Hold the top part of a push-up, on the toes, arms straight, keep the whole body straight. Count to 15.)
Burpee/squat thrust 2x15
Once that's easy, we can progress to a free weight-focused program. As a teenager, this will be a great way to build basic strength and conditioning, and prepare your body to handle a more challenging plan.
For nutrition, check these articles:
Most days of the week, try to have the foods in that second article. This will be a bit easier if you can cook a little or at least prepare some of your own meals (make your own lunch, make yourself breakfast on the weekends, whatever.)
In general, eat "good" more often than you eat "bad", but don't stress out at all over some junk food. You're an underweight teen who's starting to exercise. That's a pretty damn good combo to get results, especially if we get you off on the right foot.