Actual selling of a service or product is not necessary to make some extra dollars online. Big companies are willing to pay you to bring initially non-paying clients to their website and give in as little information as their email. These big corps can afford to invest in what we call “leads” (people who sign up for free). They do this because they know their marketing is strong enough in the back-end (after they signed up) and they can get much more than what they paid for.
However, you will be dealing with brokers, who simply connect these big corps with you. You can choose free or paid offers to promote and you can make bank pretty quick.
CPA simply means “cost per action” - you get paid per a desired action people you bring do (for example signup).
In order to be able to market CPA offers, you need to sign up with a CPA network like Neverblue, MaxBounty, CPAlead among others. However, it's not like an email service where you just sign up and your account is instantly created and accepted. Serious CPA networks make sure to review your blog or website to see if you are “good enough” to promote their offers. While it may sound like a turn off, do not be discouraged. They only want to make sure that your website has decent content, is not spammy or does not discuss about things which are against their Terms of Service (like porn).
Once your website has undergone the review process, they will email or call you to talk to you a bit just to check if you do have some basic knowledge of CPA and the rules. They may also ask you about the promotional methods you intend to use for their campaigns. There are some types of traffic they don't accept and they will tell you about it.
Stick to the big names of the CPA networking world. Do not go with shady, new networks who offer attractive pays per free signups, as they might not pay you eventually. A simple search of the company name will come up with reviews about the network. It's in the interest of the big networks to pay you so you bring in more leads for their clients.
If you've got any question or doubts about the Terms of Service, do not hesitate to contact your affiliate manager who is here to assist you with all you need to know about the rules and the CPA network. Affiliate managers also give good advice on how you can improve your campaigns. So make sure to contact them!