You should start by working with renders. You can get renders easily from planetrenders.net.
If you sign up you can get all the renders for free and I'd say just about every person on this forum has used planet renders at one time or another.

Sig wise your gonna want to bring your sizing down a lot. WHen you are still learning it is better to have smaller sig sizes cause then they look fuller easier. I would try to stay around 360 x 130.

Not bad though. Looks like you have some nice ideas on the backgorund and stuff, you just need to read some tuts to pick up the techniques they use.

My first stuff was much worse than this. for sure.