Have you taken a look at some of the tutorials here?
Tutorials/How-To

There are some excellent points on getting you started with things like coastlines, the right location of mountains (like what are mountains anyway) and some very good step by step tuts on coloring etc.

I thin it helps to work from big to small, not getting caught up with the details too early.
Also, just open Google Maps or Earth and get inspired by what Earth looks like