You need to do some home-work for this first as you have lot of doubts regarding magento2 and its theme development.
You can check magento documentaion for complete details and link provided by @price in comment.
You are beginner do here are few things you which answer you questions and you can proceed further:
- You can create new theme from extending LUMA theme. Follow this
- No need to do anything in ADMIN for designing/developing front-end theme. In admin you only need to activate your
developed-theme. You can do by simply going to
ADMIN-> Content-> Design -> Configuration -> Edit. Documentation for more details.
Explaining more about
create new theme: You can create new theme by following above provided link and only copy files which you need to change/modify. This is best practice for you at this level and this is how most of theme get developed by extending LUMA or BLANK. You should start with extending LUMA as mention in post.