While joining affiliate programs is a free opportunity, it's a business – and like any business, it will cost some money to start and run it. But those costs will be associated with building, running and promoting your blog or website. There are many tools – free and paid - you can use to assist you with all of these processes. I list a few of my favorite affiliate marketing tools here.

Look for a gravity score of 30 or more, because these products have a proven track record of selling well for a number of different affiliates. Products, especially new products, with gravity scores under 30 may work but are more risky. Gravity scores of greater than 100 mean the product’s popular. You could have competition, but don’t worry about that. The important thing is that there’s lots of demand.
If my piece of content is so unique and valuable around hiking backpack recommendations, that other reputable outdoor websites are willing to link to it and build the page’s authority, then I’d have a very real opportunity to rank high in organic search for these search terms (meaning, my page will come up first when someone searches for hiking backpacks).