I've just read an interesting post over at WebProWorld about finding the right keywords and what tools to use in order to find them. Since "the beginning of time" the Overture Keyword Search Tool has been the one most people used because it was free. The results it gave was somewhat over-estimated but it gave a good indication of the volumen of searches performed.

Now there are several tools that will help you research your keywords and help you find more related terms. Some of them are not worth the time but there are a few really good ones and the best part is that some of them are free to use.

Surely you'll get even more data if you decide to pay for it but most of the free tools will do if you're just starting out. Go read the thread at WebProWorld and see if you can find a tool that will fit your needs.