What should my stance be (in tennis) while waiting for the serve?

Face forward, open stance, get low so can give yourself a little more pop on the first return. You may want to start standing on or very close to the baseline for the first few points and gauge the power on your opponents serve, if its a big one stay where you are, if its a weaker serve then take a few steps forward, but try to avoid standing far behind the base line even against big servers.

Open stance, I would stand on the corner of the alley closest to the court (theres a word for that line.... forgot sorry :/ and i hope you know what I'm talking about). I personally think standing sideways is a bad idea because it is harder to reach a ball if it goes the opposite side you're facing so I usually face forward

Your dad needs to stop giving you tennis "advice" and hire a real coach. Stand sideways?! Look at the pros -- do you see anyone doing that? Face the server (that's where the ball is coming from, right?) and try to anticipate where the ball is going based on their body mechanics. Or just watch the ball, and turn/move toward it after it is struck.