Irwin32's advice is what worked for us. We joined a club for about $150 a month and then sailed almost every week for the entire year. We started in 20 footers (and got the first ASA sailing certifications) and then moved up to 27 footers. For day sailing, we'd normally just take out a Capri 22 or Cal 24, depending on conditions.
Don't be mislead if you look into "renting" boats and find that everything is $150 for four hours. We'd never have gotten into sailing if we had paid that every weekend. Find a sailing club that has classes and boats that you can use for a flat rate. I'm in Portland, Oregon, but a friend of mine in Washington, DC has found a similar club for the same price or even less than what we paid.
Finally, don't be too picky about what boats they have. If they only have some old Rainbow 24s or Catalina 22s or Santana 20s, that's fine for a start. Even the oldest boats we've sailed (including the '67 Cal 20 we restored) had a lot to teach us, and if you don't own the boat then you'll just be moving on anyway. When I was younger, Lasers and Vanguards were fine, and some still argue (well) that starting with dinghies is the best start for the long run. Nowadays, I personally prefer keelboats, but I'm glad I started on a 20 footer.
Let us know what you find out.