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.
Many affiliate programs are run with last-click attribution, where the affiliate who receives the last click before the sale gets 100% credit for the conversion. This is changing. With affiliate platforms providing new attribution models and reporting features, you are able to see a full-funnel, cross-channel view of how individual marketing tactics are working together. For example, you might see that a paid social campaign generated the first click, Affiliate X got click 2, and Affiliate Y got the last click. With this full picture, you can structure your affiliate commissions so that Affiliate X gets a percentage of the credit for the sale, even though they didn’t get the last click.
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.