You seem to have several layers of confusion:

1) being gay cannot be a sin even in the most hard core Christianity because BEING is not ACTING. BEING somebody clearly cannot be a sin. I would suggest you think about this and spend some quality time with yourself thinking about it, because even if you eventually drop your Christian beliefs and even if you eventually stop thinking of "sins" (which do not exist anyway and are a construct, as Christian theologists would tell you anyway - they are a construct), the distinction between BEING and ACTING will still remain vital for you throughout your life.

2) "I think I might be Bi but the struggle is that I am Christian so part of my beliefs are man and woman should wed which is where I get confused again.

I know I won't marry a girl "

there is no issue or confusion for you here: you have a narrow belief that man and woman should wed; you do not plan to wed another woman; so you are staying within what you allow for yourself based on your belief. There is no conflict and no confusion.

3) "So a few years ago I had my first kiss but it was with a girl.
I grew strong feelings towards her and started becoming confused with my sexuality (I am a girl by the way)."

that sort of uncertainty appears to be fairly frequent among teenagers and may then continue into later life or may not; there does not appear to be much to do about it other than wait and see