But I don't know which one will fit please help me!

If it's a flat top I believe you'd want a B5 but you may want to wait for some more imput from the other members. I can't remember what kind of bridge a T-15 has.

Hi slammysosa, and welcome to the forum; you'll like it here! Is this your current bridge?
http://www.gretsch-talk.com/forum/members/s-macp-albums-posts-picture4566t-t15bridge.jpg
Assuming you put a Bigsby behind that, I wonder if the strings might not struggle to clear the back end of that to get to the bridge inside?

well heres the thing, I actually have to replace that bridge so do you know of a bridge that will replace this one?

There are plenty bridges to choose from - one that fits into, or covers up, the holes left by the existing one would be ideal; if there are any other considerations you have - eg, individual saddles, adjustability - then they'll help you choose as well.
Is this a job you're going to do yourself?

I was planning on doing it myself, but It seems like I may need some assistance...