Eeyore has a lot of good points. But first need to know exactly what you want to photograph. When you are using the 18-55 do you want to zoom in more or out more? That simple question might point you as to which one to buy first.
ephotozine -
Digital Camera Reviews and Digital Photography Techniques - is a good online magazine for more general photography otherwise there are several good magazines that will help you out with F stops and what DG compared with DX stands for. (and why that little red L gives my partner the excuse to go shoes and clothes shopping!!!!!)
The other thing to do is get a book and try some settings to see what happens - play time!
It seems very daunting at first but don't be put off.
Jason