When you're learning, put the seat down nice and low so you can touch the ground with both feet. Later, when you're comfortable, put that seat back up so you can straighten your legs out a bit more (though not 100% straight) when you're pedaling -- your knees will thank you. But, for now... nice and low.
Next step: take the pedals off. The left and right pedals are threaded opposite one another, so the old adage "righty tighty, lefty loosey" will NOT work with both of them. Instead, the idea is "back off" - meaning if you're facing the bike and the handle of the wrench is pointed straight up, then rotate it toward the back of the bike to take the pedal off. You'll see that if you're on the side with the chain, that means you're going counter-clockwise to take the pedal off. And if you're on the side without the chain, then you're going clockwise to take the pedal off.
The idea here is make it so you're not worrying about pedaling and balancing at the same time. Just coast around on it, figure out how to balance when you're going straight and how to turn, figure out how to work the hand breaks, etc... I'd recommend a pretty flat surface, so you'll only be going as fast as you can push off with your feet. When you're feeling a bit more comfortable, then I'd take your bike to a place with a small incline and go down it.
Like this:
http://www.youtube.com/watch?v=p9c1Upl7To8
Followed by this:
http://www.youtube.com/watch?v=07iRBitQlMY
Then, when you feel like you've got your balance when going straight and turning and can come to a stop safely, stick the pedals back on. Start off by standing over the bike, one foot on the ground and one foot on a pedal that's sticking out toward the front of the bike. Push that pedal down and the action will start to move your bike forward... If not, try moving the pedal to a slightly different position, until it feels natural. You can also use your foot that's on the ground as a way to push off (just like you did before you put the pedals back on) if it helps get you started. Or you can start off on a gentle slope to help get things going. Then just glide... Don't panic and try to get both feet on the pedals and start pedaling superfast. Just glide...
At some point, you'll feel comfortable with the start, shift your "pushing off" foot that was on the ground onto the pedal.... And, whenever you want, use it to pedal too.
Hey, look! You're riding a bike!!!