This is the million dollar question.
Read everything you can on free forums and websites concerning affiliate marketing - not the hype stuff - but real information from those in the trenches.
There are so many ways to get into affiliate marketing. And several strategies to use while promoting affiliate programs.
You need to find a niche product that is "evergreen" always in demand. This means people are always searching for this type of item.
Pick a promotion strategy: information website, blogging, article marketing, newsletter, PPC, etc.
Build a niche blog or website that focuses on this one niche.
Promote that site a lot.
Then, stick to it.
In time, you'll start to make money, if you picked a good niche, and you're comfortable with the niche and the strategy you chose.