To me, the answer to this question is simple. Join affiliate programs that market affiliate products you have interest in and can write about. For instance, I joined an affiliate program marketing health beauty and skin care products because I was interested in their acne treatment product which was very good. I researched online and offline about acne because I wanted to write what was factual about acne. Selling the product was only an addition to the article since my primary target was those who wanted information about the skin condition.
Therefore, to determine which affiliate programs to join as a writer,
1.Choose what to write about
2. Search on the internet for affiliate programs offering products related to what you want to write about
3. Make a due diligence on the ability of the affiliate programs to pay affiliate commissions as and when due to assure you that you will be paid when you have made an affiliate commission
4. Apply to join the affiliate program, get their confirmation email and become a member
5. Go to the homepage of the product you are interested in and read about and understand it and what it does
6. Write your article and put links to the products in your author signature. However, most articles directories will not publish your articles if you have affiliate links there. You may check the first article link I have above for what I did to have my links published.
There you have it all. However, if you still have questions or you need clarifications, make it known by a comment and I will attempt to answer it in a comment or in another article. It has been a long article and I thank you for reading thus far.
This is hoping that you will adhere to the advice and seek and discover how to earn more money with your articles for you deserve it.