Congrats on the tank! If there is not hard water deposits on the glass (white haze or scale buildup) then I would just use a weak bleach solution and then rinse, rinse, rinse, etc. After that I would fill the tank up and be sure there are no leaks, and put an overdose of dechlorinator to be sure the bleach is taken care of. Then I would empty it and then put in the substrate and start setting it up.
If there are white deposits then vinegar and a razor blade will take care of it.