Photoshop is probably the most widely used software to do what you want. There are some who prefer Gimp even though it is basically an Opensource free to use photoshop clone. (Not exactly fair to say I guess because it does have some things PS does not and vice versa but whatever). Both of these are raster based graphics programs which is slightly different than Vector based software. Gamerprinter here, for example, uses Xara extreme for all his work and that is a vector based product. What's the difference you my ask? Well, imo, which is amature: Vector is always scalable which can be a great thing and it has clean lines. Raster on the other hand is more like pushing paint around on a canvas. It looks to me a little more artistic. The real technical differences you can look up on a wiki or something but in the end it depends on what your style is and how you want your stuff to look. We have a thread here about software.

It also matters on what you can do yourself as most artists will say it's not the tools that make the finished product. While you can use the filters and add ons of a lot of the softwares to create some really nice looking effects there is something to be said for being able to just do it all by hand. From what I have read and seen most artists use a handful of brushes at best and just go to town. That said, there is a reason Photoshop is the most widely used product.