Making 12V DC from 6V AC
First when you rectify 6 V AC using full bridge rectifier, you'll get 8.4 V DC.

If you use doubling rectifier like those on images, you'll get about 16.8 V DC


If 16.8 V is too much then you can drop is somewhat using voltage regulator or so.

I even suggest you to use regulator to have constant brightness.

ADD

It is also good to use boost regulator like MC34063. Just follow its datasheet, while keeping in mind you'll need external transistor, diode and inductor rated not less than 10A. And do not forget rectifier before boost.

ADD2

There may be found that your generator cannot deliver enough power for normal 12 V bulb. In this case you'll have to get more powerful generator or try xenon or LED light.