Don't just focus on 'cool' effects, and if you only want to start making signatures for forum gold, you are heading in the wrong direction. Learn the basics first. Get comfortable with photoshop. Post your progress and get feedback

Study some principals and elements of design as well figure out what works and what does not.

Photoshop is not only about signature making although not a bad to start but don't restrict yourself to them entirely. You're not going to learn as much and the next time you move to another canvas size you're going to be completely lost.

My suggestion is play around with various sizes of work as well as different compositions. The more you practise the more you will discover. Its all going to come with the wanting to learn and the patience that comes with that.