Welcome to RS!
I agree that a 12 gallon tank can be done successfully but better done by someone whose been in the hobby for a while and knows the demands of such a tank.
If you are willing to put a lot of time in the tank, then I say go for it.
Personally my first tank was a 10 gallon, and there were a lot of pros and cons to the system. Pros- ideal for the reefer with a lack of space.
Cons- Absolutely no room for error with water quality, and for most people the major con is lack of room for fish. Con, if you decide to upgrade to a larger tank, it means throwing more money into the tank, lighting, and filtration all over again.
Just a pair of clownfish would do just fine in a 12 gallon tank...but the main problem is that you inevitably will catch the SW bug, and want to upgrade to a larger tank anyways.
Buy as large of a tank as you can afford, and the best equipment out there, it will save you a lot of money and fish lives in the long run. But first, buy a few SW books, and read read read. Nothing good in this hobby comes when things are rushed.
Good luck!
Jason