get a hold of the mel scripting for beginners book. It's very good at giving you the basics.
Otherwise all you need to do is to open up the script editor and watch what is output by maya. These are the building blocks of your mel scripts. Also have a read of the docs. The mel section there is quite good. Best advice I can give you is to set yourself a project and see if you can get it working: e.g. try to write a script that randomly places a randomly scaled object on the surface of another (or something like that)
Technical Director - Framestore
Currently working on: Your Highness