Login page/Main page for index

I have a question for you regarding website building. I asked you also on Codefigths but I do not know if you saw my messages. I have just started to use web technologies and I am trying to make my own website in order to get used to these techniques I wanted to ask you: if I have a login/register page and a main page, which one of them should be the index page?

Should I make the main page the index one, and in case the user is not logged in to redirect tot the log in / register one? Or the login/register one should be the index and, in case the user is already online, it should jump to the main page For the moment, I made the second option, but I am not sure if this is the right option Thank you for your time!