Use your brain to think about what kind of things people would search for. You can use the overture and google keyword tools for some extra ideas.
When you've got a decent list, go to the
free keyword tool at SEO book. They've got some nice keyword suggestion tools their as well, but I really like the longtail generator.
When you've got a nice list, you'll need to group them into relevant adgroups of around 10-25 keywords. You want to make sure the keywords are relevant to the ad copy as well as your landing page, for QS issues. Adwords editor can be a huge time saver when grouping keywords and writing ads.
That should get you on your way, and all of those tools are free, so good luck.