[TYPO3] Creating your own extension

[Brian]

Hello all. I have been reading the Modern Template Building 2 & 3 to create your own extension and now am in the process of creating my own. I believe that I have the backend components set up correctly but I'm not exactly sure how I can get the extension to display content in a particular way. Do I need to use a template (html) file or is there PHP that I need to parse?