So, I'm 27 and have never ridden a bike before. Now that I've lost (almost) 100lb I've decided it's just about time to learn, and Im getting myself one this weekend. One problem....I'm terrified. I have no idea how to begin to learn how to ride one of these things!

Any suggestions? The bike I'm getting is 26" because apparently I'm that long of limb to need a big one.

Definitely everything Crystal said. Also, it can be easier to keep your balance when you're not going super-slow -- going too slow can lead to over-corrections. (Think about trying to walk with a very full cup of water -- if you're watching it slosh one way, you might overcorrect and have it splash out the other way!)

Find a cul-de-sac or dead end street that has a slight slope to it. Not a steep hill or anything, just something you can roll down and peddle up without too much speed going down or too much difficulty going up.

Practice steering by rolling down the incline, and practice pedaling by riding up.

It's actually easier than going on a flat surface, because you can focus on one part of it at a time and not be bombarded by trying to control everything at once.

Personally I try to ride mostly on flat ground or downhill, as going up hills has me winded in seconds flat, but once you get good at biking, especially if you're still losing weight and/or have hit your goal, riding around up and down hills and on flat places will probably keep you in shape and increase your fitness awesomely, which is a total bonus because it's also fun and a good way to get around without paying for gasoline or having to sit next to someone who smells weird on a bus.

Training wheels, used too long, will make you dependent on the trainers. If you have a friend who can run behind you with a hand on the back of the seat, that works well too.

Normally the helper holds your seat from behind, and when you fall a bike, you normally fall sideways.

Interestingly, you can Google learning to ride a bike for adults, and find some pointers.  I noticed this:  http://www.bikeforums.net/archive/index.php/t-142307.html - which has the experience of a 50+ year-old who learned to ride a bike.  It might be helpful, because I think it is a different experience for an adult.

So I found out going down gently sloping hill is enough, going down a steep hill is maybe a bit much on my first trial run.

I havent fallen off yet, so thats a start!

Everyone is offering great advice, but here's one thing that everyone seems to be forgetting: Well done for trying to learn this, and very good luck.

Well done with all your efforts thus far.

...I was very close to giving up, but you have given me the courage to try again. Thank you.

I havent even pedaled in a forward motion yet. Im having a hard time because my fat thighs are getting in the way of me pedaling....its really frustrating.

Off centre? How? As in, the saddle's not straight, it's pointing off to one side or something?

Got a link to the bike you went for? I'll be going out with teh gf on Saturday to get her one, she's hoping to find one of these or these. I think she got sold on the idea after we got her a Boris bike a couple of times in the spring and went trundling around the Royal Parks in the sunshine :-)