We have a tutorial thread made by our very friendly members which will answer to most of your starting questions. You will probably want to use GIMP for starters, since it's free while being one of the most powerful drawing and editing tools out there.
GIMP for those who strive for an open source world filled with liberty and ideals. A world governed by shared knowledge.
Photoshop for those who strive for a world controlled by the plutocracts where your liberties and money are taken from you for the sake of the big corporations