There are many ways ... Just promote them using a website or social networks

http://USERNAME.VENDOR.hop.clickbank.net/

Do you mean you want to promote other people's ebooks, and get paid a commission on sales? (this is known as an affiliate)

Or do you want to write your own ebook and sell it through Clickbank? (this is known as a vendor)

Here is the About Affiliates page on Clickbank:
Promote Products - ClickBank

And here's all about how to be a vendor:
Sell Products - ClickBank

ClickBank is now used by over 10,000 web businesses and 100,000 affiliates to deliver their products and services instantly over the Internet, with Ebooks and software the main purchases.
Hedging plants

To promote someone else's eBook = FREE

To Promote Your Product(s) Requires a Vendor Account, and an initial product launch is $49.95....drops down after the first product for every product you vend thereafter!