Fill your website with content that will be useful to the people who are interested in your niche, and place your affiliate links throughout in the most relevant places. That way when someone interested in your content clicks your link, they go to a sales page for a product that they might also be interested in, and if they buy it you get a commission!
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.
Upselling is perhaps the greatest way to make more money and it is easiest to do with existing, established customers who know and trust you. Upselling is not about selling people things that they don’t need, it is about convincing customers that there is added value in a better product. Treat your loyal customers well by selling them your best products at your best prices.
For example, if I talk about how cool a product is, and then you find out that I’m an affiliate for them, wouldn’t you as a conscientious observer become skeptical as to whether my information is biased, if perhaps I’m only saying how cool something is because I can get paid for it? Wouldn’t that make you question my integrity with other things I say as well?