How to start designing a site template
First: define what you mean by "site template".
Both videos will more helpful to you to understand about site template -
If you are technical then it's very easy to create template design but if you are not tech person then I would like to suggest you use an online tutorial or connect professional service provider which can help to save your time and money.
If you do not want to use the template for the site, you can use a ready-made solution.