For creating atemplate you need the Atlassian Plugin SDK.
Downloads and installation instructions are available here.
You won't need to read all the SDK docs. To develop templates you only need to correctly set up the SDK. The single required SDK command is explained below.
You'll also need a text editor for opening XML, CSS and HTML files. If you are using Windows you may find NotePad++ useful.
Once you installed the SDK proceed with installing the base plugin.