Set reasonable expectations for earnings. You've only invested $20. You're going to make 5 percent on most products. That means that you need to sell $400 worth of stuff to make back your investment. You get credit for purchases customers make while at Amazon besides just the product you linked to, so it's not as hard as it may sound. It won't make you rich, but it's not hard to be profitable, and the income builds over time.
Music may perform better than books and other products, mainly because you can listen to the clips of an entire album in roughly 10 minutes and get a good enough feel for it without buying it to write a short review. If you have another topic that you're passionate about, great, but make sure you have a unique angle on the topic. People can get reviews about a lot of those consumer products anywhere. You need to give them a reason to visit your site.
However, like anything else truly worthwhile, apps require a significant investment of your time or money upfront. If you don't have the skills, then you have to hire someone who can assist you in creating a great app. But first you need to come up with an idea that will sell. Do the proper market research and analytics to come up with the right app.
Freelancing is the next best thing to being paid more for your full-time work, because professional work always pays more than unskilled. To find opportunities, let former colleagues or other personal connections that you’re available for freelance gigs. (Here are some ideas on how LinkedIn could be useful for that.) Or, post on marketplaces particular to your field. For instance, Mediabistro, a journalism site, allows freelancers to post profiles of their experience and services. Though these are more up to chance, designers can bid on jobs at 99Designs.com or submit a design at Threadless, to see if it will be crowdfunded. Elance-Odesk also lists many freelance opportunities, and you can also post your own services on Fiverr, although some freelancers say these services create a race to the bottom on fees and so are not very lucrative. If you're new to freelancing, here's how to set your rates, and here's how to negotiate raises with clients.