I need advice on a flash setup for my camera. Any suggestions?

I have a Canon EOS Rebel XSi with a 18-55mm IS Lens & EF 75-300mm f/4-5.6 III Telephoto Zoom Lens. I would like a flash, but I do not know which one is for my purpose. My photography is for a hobby, but I do not to produce great pictures. I have just got this camera a few months ago, so I am very fresh and do not know all the ends and outs. I am by no means a professional, and I am not looking to have the best of the best. I just want something that will give me even pictures than what I am getting now. What is a diffuser? What all would I need to purchase with this flash setup, such as batteries and other accessories? Any advice or suggestions are greatly appreciated.