My keywords are my targeted nitch. Such as, my wifes forum is about digital pictures and cameras. So her keywords are digital camera, sony, nikon, canon, digital pictures, adobe photoshop,,, Think about what market you are trying to tap into. From that market should come your keywords.
You should use the google adwords keyword tool. If you enter your keyword or main keyword phrase, you will find many related key word phrases, and you should incorporate those into your webpages is you can