simple help about PHP or so I think

I need help on this, I’m breaking my head and I’m very new to this.

I just know that I have to put the function on my child theme.

I do not know if it’s okay to put this in the wordpress forum but my problem is this, I want to put

if the user is logged in who does not see the same initial home, I would like to know how to redirect it, that is if it is logged in if you try to access http://www.mypage.com redirect it all the time to http://www.mypage.com/main

Is there a plugin that does that?