How am I able to start learning source 2 mapping for s&box without having s&box?

So i wanted to learn how to use hammer for s&box before it will be released but i don’t really know how to get started.

SteamVR, Half-Life: Alyx, and Dota 2 all use source 2 and let you access the source 2 hammer, HL:A is the most up to date but costs money