Git means bastard - means the same as "get" = "beget" etc.
Fag means cigarette... have fun with that one!
Here are two sites that look promising: http://www.effingpot.com/ and http://www.peevish.co.uk/slang/
Hehe we will help you with your British slang grammar! To be convincing, don't mix regions - like, don't say "I'm going up the apples and pears to put the bairns to bed" cos that would sound really stupid!