Before you start to think about the content of your site, you need to know if the words you'll be using are going to be searched for by anyone. This is a common problem on many Web sites - if you don't use the right words, no one will find you.
You will more than likely need to invest in software to determine how people are using your keywords since most free tools are no longer operational. If you have an AdWord account or a Yahoo Search Marketing account, you may be able to get insight for natural search by using your keyword buy accounts.
Here's an example: my site is about employee policies. I've written my content around that term because that is how my corporation refers to them. Yahoo's tool says that employee policy only gets 481 searches for that month. Not very impressive..
So, next I do a search for personnel policy, and I see the searches are higher (931 searches) and maybe I'm on the right track. Then I notice that my first search said "handbook", and I do another search, and find that 7,181 people are searching for employee handbook:
Now I know how people are searching, and know what keywords I need to use in my text.
Resources:
RankWrite.com, a premiere source on writing for search engines.