It's never too late for a lifestyle change! Welcome to Bike Forums. Don't be ashamed of your weight, I highly recommend stopping into a bike shop as you can usually test out different bikes before you make a decision. If they laugh you out the door, go to a different one and never step foot in that other one again. Nine times out of ten, they won't laugh or make fun of you because truth is as a small business they need your business to stay afloat (the only reason they would turn you down is if they have no business sense). You may be heavy now, but at least you are off the couch and looking to change, that's nothing to be embarrassed about! $500 is a fair price for a entry level bike, you'll find hybrids to be a bit on the cheaper end compared to road bikes which is why most beginners start out with them. You shouldn't have a problem finding a rigid frame, past year hybrid for under $500 at your LBS. One recommendation I've seen on this board for heavy riders is finding a bike with a high spoke count, reduce your chance of folding a rim.
Once you find the bike you are comfortable on, ride the hell out of it and HAVE FUN! Remember, if you're not having fun on the bike you're doing it wrong.