12/8 is going to be conducted in 4 (that's what 99% of conductors including myself would do).
However, it's still going to have that triplet sub-grouping:
1-2-3 ....................1
4-5-6 ....................2
7-8-9 ....................3
10-11-12 ................4
The numbers after the ellipsis, these are going to be your steps, corresponding to the beats. (Sorry the spacing kept messing up so I had to type it like that...)
To try to think/play accurately, you need to be thinking triplet 8th notes and have that ticking in your head. It sounds hard but just try to match it up w/ what your conductor is doing and how the beats correspond to your music.
Hope that helps?
and how fast is it? If you give me the name of the piece I could look it up and offer some further insight.