Once you get (preferably Photoshop) your editing program started up make a like 800x600 blank slate and familiarize yourself with all the tools and hotkeys/shortcuts.
Also, once you get used to it download some brushes online and play around with those and different effects. Once you're ready to make your sig I'd make the dimensions about 400x150 for it, mainly because so many people have HUGE sigs and it clogs up the forums and annoys everyone.