AGK is a BASIC variant. This means it's not "basic" per se, but just modeled after other languages that call themselves BASIC. The complete command list is in the help menu. Those are the only commands you can use on Tier 1.
As for Tier 2, it's a set of libraries you can use with C++. With Tier 2, you use C++ as you normally would, but then you can import extra functions you can call.
Tier 1 basic has a very limited command set, so with a few days of studying you'll have it mastered. I would recommend reading over every command in core and knowing how each command works and what it does.
Once you know the core commands, move on to the other categories as you have a need. Maybe make a few simple games to help learn the basics.