Here is an article from http://www.webandseoguru.com/blog that can be very useful.
When your potential customers search for something they will usually search by using a phrase (a series of words). Your very first job, before writing a single word of text, is to discover what that phrase or phrases are.
Let us say we want to sell flowers in Burbank and we want people to find us on the Internet. The first thing we would do is to find out what potential customers would search for. We can directly ask them and we can use keywords tools (these are tools provided by search engines to discover what people are looking for). After careful research we discover that the highest searched for phrase in the Burbank area is “wedding bouquet”.
THAT IS WHAT YOU OPTIMIZE FOR!