Of course you want affiliates with high commissions, but they should also have a solid reputation with high conversions and low reversal rates (you get $0 if people cancel after signing up). If they’re part of an affiliate marketplace like ShareASale or ClickBank you can see some numbers there. Companies likes Amazon/SiteGround are safe bets, otherwise do your research (or track your affiliate links so you can monitor their performance). Avoid affiliates offering huge commissions since this probably means they’re struggling to acquire/retain customers naturally. This will hurt your numbers (specifically your conversions/reversal rates).
Choose the product to promote very carefully, visit the site that sells the product or service directly and how that site is presented. Is the design simple, persuasive and is the product high quality, priced reasonably and useful to the audience you are addressing? Is it in line with the right ethical and moral system you want to be associated with?
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.