There are a lot of threads about this, amongst other issues, so can a Mod please make a thread with all sorts of info like this and sticky it?
There are a couple of solutions, the one that I would prefer is this one -
http://www.geocities.com/pronto4u/applemouse.html - although there's another one being batted round the forums that involves you holding down the mouse button to switch to right click, then holding down to switch to left, or summat like that

I'd recommend the one I linked to, as it's much more natural for a Mac user