Your best bet is to get hold of an app like Photoshop, or Fireworks if you can, then go from there. You will need something like that because of the layer based system you will be using in your creations. Then the background or bottom layer will need be transparent, and on export of your image, there is usually an option to have the background transparent, as I know Photoshop has one on the "save to Web" function, but on Fireworks, not so sure. Hope that gives you an insight.
Also, there are tons of tutorials out there on the net, google has loads :
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