I would recommend doing both Anders and Lisa’s fundamentals courses. While there is some duplication they complement each other well. Lisa’s L and  backwards L (can’t remember exactly what she calls it) will answer your key question. If someone says let’s play in A. Find your A note on the 5th or 6th string use the 1, 4 & 5 chords from the appropriate L as a starting point and you are in the right key (for major keys).