Don’t use the disk or cross that came with the servo, those are for fly guys. Not familiar with the Maxx, but you need an arm intended for it and a Futaba servo. Different brand servos have different numbers of splines on the output shaft. Check the one you have on the broken servo to start with, maybe you get lucky, again I don't know what the maxx has.

The arm has a screw in the center, take it out. Might take a good careful tug or two to pull the arm off of the splined shaft of the servo. Or carefully pry it, working around as you go.

Maybe someone else can clarify the arm you need, or most any shop should know, that's a popular model.

Turn the radio and RC on, with the trim on the radio centered, get the wheels straight, then mount the correct arm to the servo. Might need to use a screw that came with the servo, as they can be different too. Use the trim on the radio for fine tuning it straight.

Oh, Welcome to the hobby and the board. Enjoy!