To do a forward roll, place your feet together and squat down, placing your hands on the floor in front of you. Now, tuck your chin to your chest (look at your stomach) and PUSH with your feet. This is almost instinctive, but as soon as you are halfway through the roll, THROW your hands forward (in the direction you are going) to gain momentum to stand up WITHOUT putting your hands on the ground.
The backward roll is a little harder (to do and explain), but those with hardly any amount of lifting experience should be okay. This is all one motion, so read this through a couple of time. From a standing position, put your hands at the BOTTOM of the military press position (hands next to ears with fingers pointed BEHIND you). Tuck your chin to your chest. Now squat down and back. Right as your butt is getting ready to hit, push backward with your legs and IMMEDIATELY tuck your knees into your chest. As SOON as your hands hit the ground, push UP like a military press. If you are doing a true reverse roll, just push hard enough to take the weight off of your head as you roll backwards. Keep your knees tucked to your chest until your feet hit, and then stand up. Like I said, that one is hard to explain, and pics/video would do better.