Ok first of all: Setting up steam is documented and you should focus on getting this to work first. Even if steam is running, you can’t use it in the Editor it self. You need to launch the Game directly, otherwise Steam will not be registered. It is simply deactivated in the Editor by default (can’t be changed without source changes).
Then, if you are really a c++ noob, you don’t want to start with networking and sessions. First learn c++, then go on and learn how to make such things. They are not basic enough.
The ShooterGame is nothing for a beginner. Even i struggle with it sometimes!
The Steam Username is saved within the PlayerState class (available in BP too) under the PlayerName variable. (at least i guess that this will give you the name with an active subsystem). Changing the Session title is not possible without using c++.
The sessions system is not easy in c++. I recently created a basic setup for you beginners, but you need c++ knowledge to do this: