I'm having a hard time understanding Cube as an area effect. Specifically in the spell Thunder wave, are you in the center of the cube or are you outside it? If someone could explain using a grid or the like that would be great.

You're looking for this part of the rules: https://www.dndbeyond.com/sources/basic-rules/spellcasting#AreasofEffect

You are on a face of the cube.

You are outside of the cube as shown in the diagrams in the players handbook. If it says sphere it is centred on the point of origin.

Grid... Lets see, it is something like:

[______][_you_][______]

[effect][effect][effect]

[effect][effect][effect]

[effect][effect][effect]

That's perfect, thank you.