how to fix leaking shut off valve
The pipe coming out of the floor is PEX, and its actually expanded and slipped over the brass fitting to form a permanent connection. This kind of fitting shouldn't leak are you sure the drip isn't coming from between the Valve and the Brass?  (The 'white plastic' is Teflon tape wrapped around the brass threads.)  The only way to tighten it is with two wrenches: one to hold the brass and one to turn the valve.