San: You don't really need to know if you're a F or a T. It's okay to be in the middle as well.

The main thing with MBTI is to help understand yourself and based on that, how to make interactions with other people smoother. In fact you can think of having no strong preference as a plus for you. It makes interactions with either type (T/F) easier because you don't have natural bias. But if you really really want to be one over the other... think to yourself which do you like more? If you believe strongly in it and practice, your function usage will start shifting and you'll be what you want.
Well the thing that really kicked start Ti for me are discussions here on MBTIc and INTPc from before. The atmosphere in INTPc is much more Tish... perhaps you can try inserting yourself there. Also try logics puzzles and just explain random things logically to people. Let Ni make connections between two things and then explain how it arrived from point A to B.