Photoshop is a good one to learn to use I think, not necessarily because of the quality you can get out of it but because there are also tonnes of tutorials for it. I have only really tried photoshop 7 so yeah I don't really know how good it has gotten

But I use GIMP, another programme that was meant to be used as a photo manipulation programme, however it is still relatively decent and I think it's a lot less daunting to use. i.e. because of all the buttons and state changes etc that you can fiddle around with in photoshop and also another big plus is that it's a free programme to download.
But yes, photoshop is a good resource, if you are patient with it. I would also recommend buying a tablet just to make drawing into the computer a little bit easier for yourself. However to get a 'decent' one you would have to fork out at least £100, in fact that's pretty cheap for one

Wacom is a good brand to look for in a tablet, but you know they aren't ridiculously necessary. It's just a bit more difficult to use a mouse.
Hope that's at least a little bit helpful