You don’t necessarily have to have a huge site or lots of traffic. Consider emailing an affiliate program’s contact person (look for contact info on the site or in affiliate newsletters) if you send a lot of leads their way, rank well in the search engines for a related keyword or have a high conversion rate. Make your email compelling. Read my tips here. You just have to be a good fit and provide excellent value to the merchant. Another good resource for this is here.
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.