As far as I know, you need to have a website or social media account with high traffic, especially in gambling niche, and there you can start promoting casinos that do have an affiliate program by creating content that later when your website visitors read it, they can be interested and register via the link you have embed it in the content.

It will be easy if you already have a website that has a lot of visitors because casinos will easily agree to your request, but not all casinos give good returns in this affiliate program and the commission you get really depends on the activity of people who register via your affiliate link.

Because you still don't understand or new about how affiliates work, it will take time for you to finally be able to earn from this system, but if it is running, this could be one way to earn money from gambling, everything is almost the same as how to make money on the internet, sometimes it can be successful but on the contrary, it doesn't always provide a good income.