A good place to begin is Google Adwords. You'll find a link to their very good tutorial on the home page at Google.com. Look just below the blank where we type our search strings and you'll see an often-overlooked link that says "Advertising Programs." Click it. It's the first step in a journey. After you've used Adwords awhile you'll be contacted by brokers selling banner ads, co-registration opportunities and a whole host of other things far more expensive. Good luck to you.