Usually you have to find some low cost keywords in your niche, then add buying prefixes. Then do more keyword research and find other keywords that you use... But there are tons of guides on PPC, you just have to search for them
As mentioned, whole books have been written on the subject. Here's one. PPC is not something you can say "do this and you'll be allright" kind of thing in a paragraph in a forum. It's not about finding low cost keywords and adding more and more keywords as the previous post suggests.