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).
Amazon runs one of the largest affiliate programs around, called Amazon Associates. You can become part of the program as long as your website already has some content (about five or ten posts). Bear in mind that once you’ve applied, it can take a few days to be approved. Once your account is ready, you can start to focus on building an effective website.
2.2 You shall market and refer potential players to the Sites. You shall be solely responsible for the content and manner of such marketing activities and the same must be professional, proper and lawful under applicable regulations and laws as well as complying with the terms and conditions of this Agreement and any instructions we may provide to you from time to time in writing.
Additionally, you must either include the following disclaimer adjacent to the pricing or availability information or provide it via a hyperlink, pop-up box, scripted pop-up, or other similar method: "Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on [relevant Amazon Site(s), as applicable] at the time of purchase will apply to the purchase of this product." In the above examples, "Details" and "More info" would provide a method for the end user to read the disclaimer. 
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