Sillz,
Welcome and so glad your here. First, find yourself a good practicing ground to ride on, like a nice flat/wide parking lot so you can get your balance and feel of the bike. Once your more comfortable, start going for longer rides... 1 mile, 1.5 miles.. 2 miles.. you get the idea. Do you have a local bike trail you might use?
You may need to make adjustments to the saddle in order to find the right position. I actually bought a trainer for fitness before I started putting in real miles on the road and dialed in the seat that way. I did a few miles, changing the pitch of the nose, another few miles, raising the height a half inch... on a on until I had it dialed in and my butt was happy.
I too am a big girl and have dropped about almost thirty pounds since I started riding in August and logged 100 miles or there about. I started out at 295 and last night weighted in at 268. Don't let your weight make you ashamed or scared to ask questions here or at at the bike shop. I hope you have a good local shop. Mine is pretty good and the more I ride and the more I learn, it seems the more they teach me, and the more I'm willing to ask.
I was so scared to admit that I was 35 years old and had never ridden a bike that I basically found the bike I wanted online, went to the LBS, plunked down my money and took home the bike without ever actually going on a test ride or being properly fitted.
My first real ride was with an instructor who was sooooo cool he really inspired me and I don't think I'd have gotten as far as I have without. It was measured in feet, not miles, but he brought out the confidence in myself that I just didn't know I had in me.
What kind of bike did you buy? You may have to try several saddles out and ask about refund policy at your LBS so you can return a saddle that doesn't agree with your bum.
What ever you do, keep putting the miles under you, it doesn't matter if your slow or out of gas after just a little while. You'll get there!!
Good luck and let us know how your doing!!
Shannon