There are various ways to make money online. Some of them do not require a lot of effort, some get more profitable over time but, generally, the better the idea and the more effort you invest into it, the more the payoff.
You may find tons of web sites promoting lines like "work from home", "make money online", "leave your job", etc. Be aware, if it sounds too good to be true, it usually is.
Get credits for different marketing options - emails, clicks, promotions...
There are sites that act as a market for individuals or companies that pay for blog posts about certain topics, products, services, or companies.
These sites offer a traffic exchange. It means you surf other people web pages and, in exchange, you can submit your web pages that other people will surf in the process.
A few more links here.