Im new to renders!

Hello, I have had blender for about 2 Months now and I had been using the Game Engine until a week or two ago, I have created some pictures and I would like to know all the tips you can give me to make them better! Thanks.

Here they are:

Explore RossBlenderArt

Please strongly advise and can you please be harsh? Thanks.

(All of these were made in 1 day (each)