“THACO” and “Keeping track of kills” are 2nd Ed conventions, not D20/3rd Ed. So is “Infravision”.
Gnerally, in 3rd ED nowadays, xps are calculated by the monsters the group as a whole defeat, then divided.
Lots of good advice so far. I’ll add a couple more things…
The D&D rules are a good place to start, but don’t feel compelled to follow anything you don’t like. That said, it is important to be consistant.
Don’t give away too much magic too soon. Nothing spoils a campaign more than a bunch of 5th level characters who all have armor of imperviousness and swords of everything slaying +10.
Similar to above, don’t be in too much of a rush to get to level 23 demi-god slaying superheroes. Unless that’s what you really want, and then, hey, it’s your game.
In-person gaming should be way more about character and spontenaity. It’s what makes it superior to computer gaming. Get into roles, make up voices.
Don’t worry about killing off a few low level types permanently, but tread lightly after the campaign’s been running for a while. Give a beloved character a chance to come back, even in substantially altered form, from permanent oblivion.
Run a side bar with intelligent monsters that respond to characters actions over time. After sneaking in through the hidden back passage a few times, expect the goblin king to figure it out and post a few guards, or a trap, or whatever.
It’s been a decade or two since I played a real table top D&D game. A few more years and my kids will be old enough though.