Always make affiliate links nofollow. Google’s goal is to provide its users with the best possible search results to their search queries. One of the main ways they figure out which webpages are the best is through links. The Google bots crawl the web, reading text. When they come to a link, it’s like an open door. They follow the link (walk through the door) and explore the site you linked to.
Be honest. Talk about what you like and don’t like. Be fair and build trust. It will serve you well later. For examples, check out my review posts about Elite Blog Academy and Self Publishing 101. I get emails frequently from people who tell me they decided to purchase one of those courses through my affiliate link because it was the most balanced review they found.
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.