Firstly, I would have assumed you have already signed up with Clickbank as an affiliate. If you have not, please proceed to their website to register.

Selecting the right product to promote.
This is actually one of the trickiest to do. Do you select a popular one and risk being in an over-crowded market? Do you select a new product? But risk low popularity and sales? How about a 'middle' product? And risk being mediocre? As you can see, it really takes a lot of skill and experience to select the right products.

Select a high paying enough product.
When I say high paying, I mean in terms of commission. How many percent do they pay you? 50% 55%? For me that is quite low already I would, generally, never promote a product that has a commission below 50%. Almost watch the price of the product, if it is too high perhaps it would deter price conscious customers.

Take a good hard look at the sales page.
The sales copy is one of the most important, in fact I would rate it the most vital, aspect to look at. Because all it matters is whether your traffic you send to the affiliate link will convert, and a good sales page certainly does up your chances a whole lot. Rate it upon 10, how convinced are you yourself to buy? Certain stuff to look at includes testimonials, bonuses, pricing, layout etc'

Look at the stats generated for the vendor generated in Clickbank Marketplace.
It is almost a no-brainer that you have to come here to look at some very important statistics. It gives information on the percentage of sales made through affiliates, how recent sale were generate and the payout. More on deciphering these statistics will be revealed in my book 'Clickbank Profit Machine'. Pay close attention to all the statistics here.