You can search Clickbank by category or by keywords. To begin, login to the Clickbank site, then click on the "Marketplace" link on the top right of the Clickbank page.
When the Marketplace page loads you will see a field that says keywords. This is where you can type a specific word of phrase that describes the type of affiliate program you are looking for. For example, if you type in 'eBay' you would get affiliate programs related to eBay. If you type in 'weight loss', you will get affiliate programs related to weight loss.
Alternatively, you can select one of the categories from the category drop-down menu, such as Business to Business or Marketing and Ads. This is a good way to start if you don't have any precise ideas for products you'd like to promote.