Welcome to the Guild Alex! Well I'm not sure I know of any software that is 'easy' to learn per se, but there are a lot of different tutorials here that can help you get started. The most popular forms of software are Photoshop and GIMP (the latter of which is free to download). Some people here also really like CC3. My suggestion would be to spend some time browsing the tutorials section to get a feel for what styles you'd like to go for and see what software is needed to create them.