I usually use coral paint shops, but if you don't have an art program like that, here are some sites you can use online...
Picnik
Fotoflexer
Lunapic
And for someone new to making banners, I would suggest using a screen shot from the show or the anime, as opposed to fanart to start with. I find the manga images are usually the easiest to work with.
And make more than one banner for what you want, using different images or fonts, it'll give you the chance to choose the best one rather than just grabbing the first one. I almost always make two different versions of the same banner, especially if it's for someone else, to give them more options.
