A tub spout may be screwed onto a threaded pipe or anchored to a nonthreaded pipe with a setscrew. Before you start to unscrew a spout, check underneath to see if there is a setscrew.
If water seeps from behind the spout, it may be attached to a threaded pipe that is loose. Remove the pipe, wrap its threads with pipe-thread tape, and use a pipe wrench to tighten it.
If possible, spend a little more for a brass spout. It will last longer than a bargain-bin spout, which may be made of thin metal or even plastic that wears and stains quickly.
Less than an hour for most repairs
Screwdrivers, hex wrench, groove-joint pliers, pipe wrench, caulk gun
Unscrewing and reinstalling a spout
There is no need to shut off the water; place a towel in the bottom of the tub to catch any debris or caulk
New spout, pipe-thread tape, tub-and-tile caulk