I didn't play bioshock 1 and 2, do we have to play it for infinite? or is it unrelated?
Bioshock 1 was fantastic, 2 was meh. They both took place in the same city: Rapture, and were loosely related to each other. On the other hand, Infinite is set in a whole new locale. You may not have to play 1 or 2 to play Infinite but I would play them anyway just so you'll have some understanding of the BioShock universe.
With Infinite, I think they are trying to take BioShock in a Final Fantasy direction (i.e. multiple games under the same name, but each with a different storyline and protagonist).