I can tell you right now that if $500 is your budget forget it.
I started scuba diving when I was 14 and it took me two years of part time work to buy my first scuba set-up. Much of that was second hand and all of it was upgraded as soon as I had a better job. Taking the course is going to cost some cash and a basic C-card is just that. You'll want/need to take at least a few other courses beyond that.
There are deals to be had on older scuba gear if you checkout Ebay, but until you take the class you won't know what they are. For a decent set-up new, of the good stuff like Amphibous mentioned, your looking at $1300-$2000 to start with. In your area a drysuit would be a very good idea. I actually went with a used drysuit early on since I only had to worry about the neck and wrist seals fitting me and I got quite a bit of growing room out of it.
Of course drysuits are much fancier, and more expensive, than the one I bought all those years ago. Still, I got at least 4 years growth out of it when I need it most.
I would say that with your current budget you could eith be a VERY well equipped freediver or a scuba diver who needs to rent every time they go diving. When I sold off my scuba gear I instantly had cash for all kinds of fancy fins that I wouldn't have bought other wise. For what I wouls spend on one day's worht of trimix diving I can now fund an entire year of freediving/spearfishing trips.
Seriously, if you want to go the scuba route I would buy some good mask, snorkel and fins (NOT split fins) and then take as many courses as you can with the rest of your cash. Rent everything else while your growing and see where it takes you. For me I went out to California when I was 17 and became a scuba intructor- turned 18 just a week shy of course completion.:t That gave me a job to work my way through college with and have done it part/fulltime ever since.
Still, If I could do it over I would have just started out freediving and used the huge amounts of cash that I spent on gear, gas fills, courses, ect. on a bunch of really cool trips instead- I bet I could have circled the globe a couple of times by now.
Jon