The best way to find suitable brands to promote is by simply using a search engine using: '[Brand] + Affiliate Program'. Some companies run in-house affiliate programs however, this is a very specialist area. Therefore, most companies opt to employ an 'affiliate network' which has already built a large base of affiliates and gained years of experience in running successful programs for clients.
What kind of commission do they offer? One-time commissions or recurring commissions? For example, many programs pay you one time for sending a customer. On the other hand, some programs like membership sites or SaaS (software as a service) programs will pay you a commission as long as the person you referred is a paying customer. Recurring commissions are great when you can find them!
Always disclose your affiliate relationship. Most visitors will probably understand that graphic ads will lead to your getting paid, but if you write a review or use an in-text link as a recommendation, you want your readers to know that may lead to compensation as well. This ensures you retain transparency and trust with your readers, but also, it's required by the FTC's endorsement rules.
One last note. I’ve been asked what I think of other affiliate marketing resources, both free and paid. I’m familiar with some of them, not all. I’ve read ebooks, watch videos, bought courses and more. So far, the only paid-for course that has impressed me enough to recommend is Kayla Aimee’s Affiliate Acceleration: Impactful Strategies To Increase Your Passive Income.
Affiliate marketing is also called "performance marketing", in reference to how sales employees are typically being compensated. Such employees are typically paid a commission for each sale they close, and sometimes are paid performance incentives for exceeding objectives. Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.