WIth the tin whistle, it is quite hard to get the correct amount of breath to start with. However, the fingering is remarkably easy - as tin whistles are key specific (as far as I'm aware), the same fingering is transferarable to all keys. One of the difficult things to learn is the difference in the breath required to play at the higher octave - you use virtually the same fingering, but just blow a little harder. It is slightly harder to play sharps and flats, as this requires half covering finger holes (harder at least than the recorder).
There's some useful info at
http://www.hobgoblin.com/faqwhistle.htm
It depends what you want to play, but I'm guessing you'll try to play Scottish/Irish folk tunes. The best bet in my opinion is to get some recordings and listen/play along - if you don't want to shell out.
SCAT (Scottish Culture and Traditions) organise a whole raft of instrument classes (see their website at
http://www.scottishculture.org/ - they also have a session in the Globe (North Silver Street) every Tuesday evening. There's also a session in the Prince of Wales on a Sunday (as far as I know - I haven't been for a long time) where heaps of folky types play - it certainly used to be the case that all musicians got a free pint if they turned up before 9pm, but with the Prince being up for sale I don't know if that still applies.
Hope that helps
Regards
Flossie