Through LinkShare, various companies make offers that you can accept if you like the terms. Other programs offer click-through payouts, usually a few cents for every individual who clicks the banner on your site and visits the commerce site. In LinkShare, you browse by topic to find affiliate programs that fit the content of your site and the interests of your readers. When you find one, you choose to accept the offer. If your site doesn't get that many hits, look for another offer from that or another company.
Some companies have different types of offers for different sites.
After a few days, you'll be approved and you can add the advertisement to your site. You can also check into LinkShare to get e-mail offers and enticements from potential advertisers.