As the owner of a 35 year old 12 string, I would also advise that you never use a heavier gauge string than a light. We could argue all day as to brand, but Martin Marquis or D'Addario are good sounding and last a reasonable amount of time. As has been said, for entry level players, tune the 1st and 2nd, and the 3rd and 4th in unison. The remaining pairs should be tuned an octave apart. Strictly speaking, Allan C's post on whether one is wound and the other isn't, isn't correct as on many sets the 9th and 10th, and the 11th and 12th are both wound but still should be an octave apart. And just in case you are unfamiliar with the numbering of strings, the highest sounding string is number 1 and as the tone gets lower the string gets higher. Hence, on a 12 string, the string closest to your waist is 1 and the string closest to your chin is 12. Good luck. These are great instruments. Start by playing it like your six, but you will find that you might have to play in different keys than you would on a six due to the octave strings. As you become familiar with it, start to experiment with different playing styles and tunings.

All the best,

Big Mick