Many marketers new to Clickbank often don't know where to start, which products to pick. There are many parameters to use, but I say start by picking a niche.
Don't randomly pick a niche. Don't pick products just because they are selling well, have a big gravity number.
Start by evaluating your passions. Which subjects are you passionate about? I like dating. Before I was married I read many how to books on the subject. I developed a real passion for it that is still with me. I give my girlfriends dating advice all the time. So, I usually pick Clickbank dating products to promote.
Why do you want to follow your passion? Because you'll be able to generate more and better content. If you pick something that bores you, you are not going to be able to sustain your marketing efforts.
Trust me, I've tried promoting FOREX products and I just can't sustain it. I don't like it. I find it boring. While I've learned quite a bit, it's just not something I can sustain.
So, get together a list of passions. You don't even need to be an expert yet. You can always develop expertise along the way. All you need is passion!
Once you have the list use a Clickbank search engine to research products. Bookmark or jot down the products you like. From there evaluate the products. Why do they pay per sale? How does the landing page look? What's the refund rate? How competitive is it on Google? From there pick the top product in two or three of the Clickbank niche's you've selected and get to work!