What to buy for my Canon EOS 500D?

That 18-55 lens is the all-around lens you've been dreaming of. If you don't know how to use it properly, you will be drawn to buy lens after lens searching for the biggest and longest ever. Before buying anything and regret later, get a good book about the basic principles of photography. That should teach you how to do things the right way with what you have right now and what you will actually need to buy later.