I am a computer guy, not a handy-man, so forgive the stupidity of my
question...

I have a Rheem 40 gallon water heater.
240 volts, 4500 wattage, model number 81V40D C ...

My dad, a water heater inspector for TVA, tells me the top element is
probably bad (We have some hot water, but runs out pretty fast now... i
trust his judgement since he has done it for over 25 years).

So I go to home depot to grab a new element, and I am seeing screw in,
twist in, walk in, fly in, run in, etc... :^)

can someone just tell me what to get and where to get it? Pretend I am
your stupid son and feel sorry for me and hook me up. My dad just says
"Buy a new hot water heater, they dont cost much" but I dont have time
for all that, guests arrive in about 10 days.... I know all i will have
to do is turn the breaker off, let the water cool, drain the tank, and
pull (or twist, or unscrew) the element out and slap a new one in
there...

However, I dont have an ohm meter.... i mean, does the average guy have
one of these lying around? Ive replaced light fixtures and ceiling
fans, but the more i read the more people seem to freak out about the
water heater still haveing power in it even after turning it off at the
breaker... what gives?

anyway, thanks in advance!
Jazz Mann