Start with a Google AdWords starter account to learn the basics. Brainstorm possible keywords related to your product or service. For example wedding planner could have an abundance of potential keywords relating to anything about the wedding: bride, groom, wedding party, wedding flowers, wedding cakes, etc. etc. Make your own list and then do your research about which keywords are generating traffic and have the least amount of competition. Hands down the best tool out there for keyword research is WordTracker. They give you a free trial run and walk you through how to use the program and what the numbers mean to you.
Then, pick up a copy of Ultimate Guide to Google AdWords (Ultimate Guide to Google Adwords)