Multiclassing is viable, but if this is your first foray into D&D systems sticking with one class would probably be easier.
And yeah, prioritize INT over every other stat for a Wizard. If the game lets you increase INT, then do so immediately. To give some examples for other classes: if you were a Rogue you would be doing the same for DEX, and if you were a Sorcerer you would be focusing on CHA.