For your convenience, Clickbank has sorted their products by categories (niches).
Pay attention to a parameter called 'gravity' for each product you're looking at.
Choose the products with gravity of between 20 to 100, which means a reasonable amount of affiliates had successfully sold those products.
As for those products with very high gravity, they are obviously very popular for a certain period, but let's avoid them if you're just getting started.
Next, make sure you research the product on Google before promoting it.
The proper way is to research it on Google, then buy the product and use it yourself -- and if it’s good, promote it as an affiliate.
You can do a quick research using these
steps: Start by typing these keywords
into the search box: product name +
review
product name + scam
See if you can find any genuine reviews, and not just some affiliate regurgitating what's on the vendor's sales page.
If you have a hard time finding a useful product review, do a search on the product creator, or the authority figure/expert that you see in the product sales letter. See if you can find good stuff about these people online.
A quick example here:
One of the top products in the e-Marketing niche in Clickbank is the K Money Mastery (a Kindle coaching program) by Stefan James Pylarinos.
If you do a Google Search on his product, you will find some good detailed reviews.
More importantly, if you do a search on him, you’ll know that he’s the owner of Project Life Mastery, a very successful self-development blog, and he’s also the protégé of world-class business coach, Dan Lok.
Ok, I’m not saying that every vendor must be this accomplished, but I think you get the idea. The least you can do is to do a proper search on the product and the product creator.
Besides Google search, you can ask around in forums such as the Warrior Forum (more for the IM niche), in your niche forums, or even relevant Facebook groups.