Plumbing Newb, Leaky Tub Faucet

I have a bathtub/shower faucet that leaks. It's a three handle tub.

In addition to this it has some water that comes out of the faucet when the shower head is "on".

Lastly, if the water is on a slow flow rate (to conserve water while showering) it can make an awful rattling like noise. It sounds like the pipes within the wall are vibrating. Any idea what this could be?

Also, I assume I need to replace the stems and seats for the hot and cold water handles. In order to remove the seats I need to turn off the water to the house. I don't have a basement (1970s home in TX). Where would I find the cutoff valve? I think it's in a hole in the front yard near the fire hydrant.

After I turn the water off do I need to open the tub faucets (hot and cold) to drain them so I can remove the seats? Do I need to turn off my hot water heater first?

I'm a total plumbing newb, but want to learn!

Much thanks in advance!!