If you've never used Photoshop, there will be a pretty steep learning curve. There are entire college courses dedicated to it. I have been using it for close to 10 years now, and discover new features/methods all the time. I haven't looked into the cost of it in a while but it's over $100. I've been using CS5 for years.
A free alternative is Gimp which is an open source Photoshop wannabe. Also Paint.net (RebBugler uses this). John B. used Paint.net for the Chancellorsville maps.
However if you are set on Photoshop, they have a free trial that I would try out before you buy.
Like I mentioned, you don't really need graphic design experience, but do need to be artistically inclined and able to pick things apart and figure them out. Back when I was first learning maps, LouiRaider (GB and Antietam map creator) gave me basic information over email, and then I developed my own methods from that. It's sort of like playing a musical instrument. You need instruction to learn the instrument, but after that it's up to you to create the music.
