Lead your Readers' Attention
A "Call to action" is a phrase or single word that gets your visitor to become active on your page. Examples for obvious CTAs are the "Buy Now!" or "Like us on Facebook!" Buttons. The first one urges a user to buy your product the latter wants to build relationships.
In order to create CTAs that work for you, you have to identify what it is, you would like your visitor to do. As a rule of thumb, your primary CTAs should be placed over the fold (that's the part of your website that can be seen without having to scroll further down). Make them stand out by using the signal colour of your website's colour palette, and by designing them to be actionable ("Need Help? Click Here!" rather than "FAQ Page").
While most CTAs will probably have been positioned when your website was designed, links can serve as CTAs in every new article and should be set up to convey the intended message and action.
There's so much more to CTAs than would fit into this article. I've recently attended a very interesting conference by Unbounce that dedicated a whole day to optimizing landing pages and call-to-actions.