There are three main venues to promote your site that use pay-per-click (ppc) marketing: Google, MSN, and Yahoo Marketing, which used to be called Overture.
We have by far had the best success with Google but I strongly recommend getting a Yahoo account simply for their marketing manual. It's over 100 pages of great tips on how to market with keywords and includes formulas on calculating return on investment, customer value, etc. It also has a section on creating a marketing plan. While the examples are written for Yahoo, the information will work just as well anywhere else.
Give it a try!