Wow, this is exciting. You're going to make sure that all the pages of the website will be using the template you just created. For this, you only have to edit the page properties of the root page. This can be done by clicking "List", right click on the icon of the root page and then click on the Edit option OR by clicking "List", left click on the TYPO3Buddy root page and then click on the edit icon, all indicated here:
After that, click on the "Appearance" tab. At "Use template design" choose the main template. To make sure that the template applies to all other pages on the website, also choose the main template at "Subpages - use template design":
Don't forget to click on one of the save buttons on top of the page. If the template doesn't show in the pull down menu, you've missed something in one of the previous steps.