try berkeley board sports. they have a better selection of men's suits, but since you're probably a man, that's ok.

I got mine at 1440 Sports, or whatever that place on San Pablo is called

yes, the most popular thickness for the bay area is 4/3 (which means 4 mm in the center of your body and 3 mm on your limbs).

i find that a 4/3 is a bit heavy during the hottest parts of the season, and i do see some people in the bay with short suits. however, i'm not yet motivated to own more than one wetsuit, and i'd rather sometimes be too warm than too cold, and i've never felt cold in my 4/3. plus, if you get a bit too hot, you can always cool off by hopping in the water.

note that windsurfing wetsuits fit a bit different than surfing wetsuits. i'm not sure how much of a difference that really makes, but mine is a windsurfing suit.

the most important thing is getting one that fits you well. try on a lot of them until you find one that you're happy with. while there is plenty of used gear available at swap meets, i recommend buying a wetsuit new.

one other random bit of advice, you'll probably sweat quite a bit while trying on wetsuits in the store. bring a bottle of water so you won't get dehydrated.

the berkeley marina is a great place to learn, but once you get your own equipment there are plenty of other locations too. coyote point, candlestick point, point isabel, to name a few. i haven't yet been to either crissy field or rio vista, hopefully this season.

Chrissy field is amazing once you get good enough to at least water start. It helps if you can jibe too. The spot is beautiful, but be prepared. The currents are strong and can really take you for a ride if the wind dies. Ask one of more advance people before you head out, they will tell you the wind and current conditions. Also, bring a spare universal, a good wetsuit and maybe a hood, and even a vhf radio or cell phone in a waterproof pouch. Be safe and have fun!
-Ben