One bit of advise, do not view this as having to be done right the first time.
Your idea of seeking keywords is the same first approach I would take. Do not be afraid to throw out all of the work you did in your first attempt. If the result is not what you seek take what you learned by doing the first attempt and start over from scratch.
EDIT
In doing quick search for some free tools to do word analysis found this information page on Text Analysis. Nice read.