You have a flip flop hub, the side with the stepped/reversed threading is the side for the fixed cog (although you could put a fixed cog on the freewheel side, it would just be unsafe without a lockring)

Google gear inch table, i find them easier than calculators and ratio generators.

How do you have almost 500 posts but don't know the very basics? I'll just assume you're new to fgss, welcome
look at the stickied newb thread