So let's get started with something small and simple. 1000 x 1000 at 100 ppi, white background, RGB, and 8 bits.
1. Create a new layer. Grab the Brush tool and choose the 5 pixel soft round tip. Make sure that it is at 100% opacity and flow and it's mode is normal. Draw some squiggly coastlines and islands. Doesn't have to be fancy, doesn't have to be done with a tablet (but it looks cooler if you do because you can get different lines widths). I always try to put a few islands at the end of any piece of land that sticks out and then a few random ones in various areas. Just so you know, I'm intending the space on the right to be the ocean and that on the left as land...otherwise my little islands would be lakes. Make sure that all islands are fully closed (no open spaces) and make sure that the coastline goes all the way to the edge. See Pic 1.
2. Put in some rivers. They don't have to be perfect and you can change them later, after the mountains are in, if you don't like where they are now. See Pic 2.
3. Put in some lakes and make sure that they are connected to a river. See Pic 3.
4. For the mountains I use a roughly triangular shape with lots of wiggles in it. Put the highest, pointiest peaks in the middle and then round them off as you get further out. See Pic 4.
5. Put a squiggle down the middle on the peaks. This will serve to show us which side is facing the light and which side is in shadow. See Pic 5.