Are the default databases necessary? Master, Model, msdb, tempdb

Hello,

I admit, I know nothing about SQL but would like to learn some basics.   WHen you set up SQL server 2012 (and previous versions) it usually has some databases when you first open SQL (probably an option during install.

Do I need to keep master, model, msdb, or tempdb?   This is a fresh install and there is no production database or database apps installed yet.