Blank screen for /admin and public site with Craft 3
What you'll want to do is change the CRAFT_BASE_PATH in your index.php file. 

Look for line 7, and change this: 

define('CRAFT_BASE_PATH', dirname("__DIR__"));

to this: 

define('CRAFT_BASE_PATH', dirname('./'));

I had this issue as well, being used to doing the same thing with Craft 2, changing the $craftPath variable.