Hopefully you bought a valve with good "flow through" like a ball
valve. The ball valves have nice easy-on easy-off (quick-on quick-off)
action, too Getting the old one off is just a twist until it either
comes loose, not likely if it is the normal plastic kind, or until it
breaks into small enough pieces to be removed. The tank should not be
harmed unless is is in bad shape already.
Yes, use pipe dope or teflon tape to seal the threads. That is
necessary for any threaded pipes, and the water heater has a standard
3/4 inch female pipe thread. --Phil

Rein wrote:

thanks for the info. I bought a brass valve and was going to install
it but I can't get the old one out easily. I am afraid I might break
something it seems to have been twisted in pretty tight.
Also, do I use teflon tape or something to seal the brass pipe going
into the tank ?

thanks
--
Phil Munro Dept of Electrical & Computer Engin
Youngstown State University
Youngstown, Ohio 44555