Update: Please review Nuno’s comment before reading this answer. This answer relates to getting content (and other items) in and out of the CMS, rather than rendering published content on your website.
As Nick says, you’ll want to use the Core Service to get content (and other items) in and out of the CMS itself.
As well as the SDL documentation link that you posted as a comment, you may also want to take a look at the Core Service Recipes in the Tridion Cookbook:
This includes things such as configuring the Core Service and some basic scripts on working with content and metadata.
There are also a lot of helpful questions and answers on here under the [core-service] tag (which I’ve added to your question).
Mihai also has a lot of information about using the Core Service with a Java client. I’m not sure how up to date this is though, as it says the last supported version of Tridion is 2013SP1:
Hope this helps!