The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.
Know when (and when not) to use Viglinks and Skimlinks. If you applied to an affiliate program but were denied, you might be able to still be an affiliate for that advertiser through a secondary affiliate program like VigLink or Skimlinks. Basically, they themselves are affiliates and will split their affiliate commission with you if you put their affiliate link in your content for an advertiser. Obviously, the commission rate is lower for you in this case, so if you ever are accepted into the advertiser’s affiliate program directly, immediately switch from using VigLink / Skimlinks affiliate links to your own.