Always disclose your affiliate relationship. Most visitors will probably understand that graphic ad 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.
ClickBank allows you to join for free, and the approval process is virtually automatic, so it’s a great choice for people entering the affiliated game for the first time. ClickBank has a ton of information, including FAQs, walk-throughs, and videos available, so the barrier to entry is quite low. There’s also a (paid) program called ClickBank University with courses and assistance from experienced marketers.
Affiliate marketing is also called "performance marketing", in reference to how sales employees are typically being compensated. Such employees are typically paid a commission for each sale they close, and sometimes are paid performance incentives for exceeding objectives. Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.