The first few of these is a good rough primer on world building:
Bat in the Attic: A Fantasy Sandbox in Detail Part I

If you have your world already and just want to map it, here's what I do:
1) A blank layer, just white.
2) A land layer. Draw your land in a single flat colour (like green!)
3) Do "select by colour" on the land.
4) Create a new layer, called sea. Do select->invert. In bucket fill, choose "whole selection" and fill it with blue.
5) Create a new layer, called coast. Do select->invert again (this will bring you back to the land selection). Do select->grow (by 1).
6) Bucket fill it, in black.
7) Go back to your land layer, and do select by colour on the land.
Go to your coast layer, and do "cut". You should be left with a 1 pixel wide coast line!

How to draw your land?
1) With a great big sized pencil, block out continents. Then reduce the size and add peninsulas and islands. Reduce the size and add in fine details.
If you want lakes, it's better to change colour, and draw them in pencil (with hard edges) and then select by colour and cut, rather than use the eraser, I find.

2) Get a map of the real world, and cut out bits. Paste them as layers in your map, resize them, rotate them, change them around, and then flatten all that out to one layer. Use that as a guide to your land drawing as above.

3) What I'm doing lately, being *super* lazy: As above, but guide nothing. That's your land layer. Maybe do an extra layer underneath it and colour in green, just to fix any holes etc. Do a select by colour on the sea region (I make all sea bits transparent on the original map so I can muck about freely), and you can fill in land and sea later. You can use the "real world" mountains and rivers as guides as well later on, but you don't need to.

Another thing I'm doing lately, with rivers, is to draw them all the same size at first. Then count their tributaries. 5 tributaries means that river gets bumped up in size (I start at 3, and go in 2 pixel increments). Then 10, 20, 40, 80. I've not gone above 80 yet...

I usually have a river layer, a mountain layer, a hill layer, a forest layer, etc. I originally used the "ivy" brush on gimp to draw forests, before making my own. There are lots of forest/mountain/hill brushes out there, lots of good ones on this forum alone.