I'm just starting at Clickbank, so I'm a total newbie, but if I want to follow this strategy can I do it promoting just one product, for example, Category>Health & Fitness ---> X product. Then, should i start a blog or something like that where i can talk about things related to the product subject? if so, do i have to start making entries about this recurrently? or I can opt for just making an article with a free ebook in exchange to user's e-mails and follow the process?
There are loads of resources for making money online as an affiliate. You could source products from ClickBank, Commission Junction, Rakuten Marketing, Share-a-Sale, Impact Radius and many others. Plus, many of the larger companies have their own affiliate programs as well. Do your due diligence and find the right company with a relevant product or service to your audience that you can sell as an affiliate.
If you're looking to address some immediate financial needs, then the app economy is likely right for you. Thanks to the global sharing phenomenon, launched in part by our smartphones and ever-burgeoning global connectivity, you could easily opt for some quick active income by using well-known apps. From ride sharing to deliveries and even quick tasks, there's an app for that today, as the saying goes.
If you have pop-ups and optins on every page, I would have to think that it would be a lot harder to convince someone to link to it. Especially since the pages where you have your optins (and probably receive the most traffic) are probably the same pages that you have your best content on as well that you'd be hoping to pitch in your outreach campaign.