A 12 string guitar is tuned and strung the same way as a 6 six string but you have extra strings tuned one octave higher ("string pairs 3-6") and extra strings tuned in unison ("string pairs 1-2")

That means (low to high):

String pairs:

6, E one octave higher than standard
6, E as standard
5, A one octave higher than standard
5, A as standard
4, D one octave higher than standard
4, D as standard
3, G one octave higher than standard
3, G as standard
2, B as standard
2, B as standard
1, E as standard
1, E as standard

..and one whole step lower:

String pairs:

6, D one octave higher
6, D
5, G one octave higher
5, G
4, C one octave higher
4, C
3, F one octave higher
3, F
2, A
2, A
1, D
1, D