Basically, for affiliate marketing you must create a professional web page / blog, gain authority, readers and then find products / services consistent with the theme of your website and above all, that you really like.
Important: I advise you to recommend a product or service that you trust and with which you have truly had a good experience. The fact of winning and being able to maintain the confidence of your followers is one of the most important things for anyone to achieve professional growth.
How to apply to affiliate marketing programs
After creating your website, you must request to join the brand affiliate program (joining these programs is free in most cases), and when you are accepted you can promote your products / services with your readers and receive a commission For each sale.
Note: generally, the only requirement to be accepted in the affiliate programs is to have a website related to the theme of the product or service you want to promote, since you cannot claim to promote articles or sports equipment, with a cooking blog.
For example: a travel blog can join skyscanner.com, one that deals with beauty can apply to the affiliate program of some makeup brand and promote their favorite products, and so on, according to each of the themes.
You can join many of the most famous brands: you just have to create your professional website and then sign up for free at cj.com, Amazon Affiliates, shareasale.com, zanox.com and tradedoubler.com, for example.