Have you tried setting the seat so low that you can just "scoot"?

Here's a good viddie showing an adult how to ride a bike. It's simplistic but it will get you started. Good luck.

Newbikie, contact me and I'll send you a description of the sequence I've used to teach almost 250 adults and kids.

John Ciccarelli (johnc at-sign bicyclesolutions period com)
Bicycle Solutions -- "Expert Coaching for Confident Cycling"
San Francisco Bay Area
See my lesson descriptions on TeachStreet.com

Newbikie et al.,
I was/am a late learner (still a beginner) and I humbly suggest, contrary to what more experienced riders have posted, to avoid declines and instead start on flat ground. There is nothing scarier than going down a hill when you're too scared to let go of the bars to pull the brakes in a safe manner. I learned on a single speed rental bike with both front brakes and coaster brakes and I think that definitely helped me learn how to start, keep my balance, etc. in a manner that didn't make me feel powerless and in which I didn't have to worry about gears. Also, the advantage of coaster brakes, despite whatever shortcomings they may have, is that it's more intuitive to use them, i.e. to push back on the pedals, than it is to get past one's fear of letting go of the handlebars. Of course, having both types of brakes allows one to learn eventually how to do use hand brakes, and once that's manageable, it's a heck of a lot easier to learn to change gears.
In the same vein, I would keep the pedals on instead of taking them off. Having the pedals allows a feeling of control, and with cbs, a method of stopping/slowing down versus just letting gravity pull you along and waiting for either friction or gravity to stop you. Definitely have the seat post low enough that the feet can touch the ground and as a very last resort provide friction/means of escape. Try to start with the bike upright and the pedals in a position such that you can generate a fair amount of momentum with the first push. I found that having my right foot/pedal up at about 1 o'clock (somewhat forward of vertical) felt and worked better than starting with my left or having the pedals in a lower position but that's a personal matter of taste and how your body feels. Once you've gotten off, you'll probably wiggle for a bit but keep your grip on the handlebars and hold them steady while continuing to pedal. Also keep your face and eyes on where you want to go until you have better control. Try to keep going straight for a reasonably long distance, at least a couple of blocks, so that you can build momentum, which will help your balance, and so that you can learn to brake slowly and safely. Don't lean! My friends, who taught me last year, made me practice going in wide circles and figure 8s around large planters and park furniture (before "escorting" me onto the road), but do practice slalom curves, as others have said. And while in the beginning it's easier to do wide turns, pedaling through them, it's better, like when driving, to slow down before the turn (but not so much that you don't have the momentum to make it around) unless you're confident enough in your balance and skills not to a. fall over or b. go into traffic as I'm always afraid of doing.