I don't have any sites which are https enabled so can't give detailed step by step instructions.
However, normally, the company through which you register your domain name will be the folk to whom to turn for help on this as each company will have slightly different interface through which to control things.
I would also expect your domain hosting company to be able help you obtain the necessary certificate. They come in several flavours, at varying costs. If you are not collecting payments through your site then the simplest form will be adequate to stop browsers reporting your site is unsafe. There are a number of organisations that offer that type of certificate at no cost. A search will find them.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.