I would recommend GIMP because it is free. GIMP can be downloaded at www.gimp.org There is a downloads section (click 'other downloads' to get the Mac version), a manual section, and a tutorial section.
Once you have experimented with that, I recommend joining up to the Cartographer's Guild (its free!) and following this tutorial on building world maps with the GIMP:
http://www.cartographersguild....ed-presentation.html (original thread here:
http://www.cartographersguild....egional-rpg-map.html ). Another useful tutorial is here (work through this whole thread all the way - the most crucial bit is posted half-way through):
http://www.cartographersguild....r-masks-gimp-ps.html
Personally, I would
not recommend any paid-for mapping software, because these are all overpriced and not as powerful as GIMP. Even though they might seem simple to use at first, they are a very expensive dead end. Only thing to watch out for with GIMP is that it is VERY resource hungry so if you have an old mac (like me) and try to make big maps it is going to be slow going.