Build a base!
Have someone who knows show you how to do Squats, Deadlifts, and Leg Presses.
These wil help you build a good base, and start you off with basic mass movements that will strengthen your hips, quads, hamstrings, and lower back. In doing so, you'll also stimulate growth in yuor upper body. For years, people have used squats as the mainstay for building true power, strength, and size.
Some of the folks here, and on the Pwerlifting Forum can probably add to what I've said.