kelownagirl "We picked up a cheap trainer at a garage sale"

wooh hooh! none of my neighbors ride

"How tightly should my back wheel sit against the metal part of the trainer?"

on mine (Cycleops fluid) lift the bike to the left side of the QR simply fits into its side, the right (deraileur side) has a knurled (is that a word?) knob that when rotated back to front is the right tension.

I have had to adjust to width whether using the MTB or road bike.

Get a spare QR. Do not use your "real" one, any trainer will well, ruin is too strong a word but mess that up.

"There are also things at the back that I think adjust the tension in some way. Should I just try them at various levels and see what feels right?"

Yes. Somehwhere here I'm sure there's discussion of whether or not to use "trainer tires". Training miles at any tension are hard on your tires.

"When you are using a trainer, is it mainly for spinning on a middle-of-the-road gear at a fairly high cadence (90-100) to get an aerobic workout or can/should you be in a high gear (highest?) and feel a good workout on the legs as well?"

You should ask someone here who trains but you can do either! Even stand on the pedals (carefull there, that might be why your trainer was in the garage sale "

"Is there a maximum speed you should go? (It seems to make more noise when I went faster.)"

Is it a wind, magnetic or fluid trainer? Wind and magnetic are noisier, fluid the quietest of the three.

Do you have neighbors close by or live in an apartment?

Do you like them? No? Then who cares

"Do you use your brakes to stop or do you let it slow down on its own? (It made a bell-like noise when I started putting on the brakes...)"

That's odd. I use either, no noise.

"Is it my imagination, or does the front wheel feel like it's down a bit lower than when I'm actually riding my bike on the flats?"

It may be lower. There is a star shaped one that has options, slightly higher feels more like climbing, then can have it even or lower.