I'd have to say your creaking and broken spoke are unrelated. Unless you have a lot of loose spokes.
Replacing a broken spoke is fairly straight forward.
Remove the wheel
Remove the tire
Remove the cassete/freewheel
remove the broken spoke
Using a ruler, measure a good spoke on the wheel or hold spokes up to the wheel till you find one about the right size. You want the threads to just disappear into the spoke nipple.
Install the spoke using the existing spokes as reference.
Tighten up the spoke and true the wheel as you normally would.
Reinstall everything you took off.
Riding on a broken spoke should be OK as long as you take it easy. Loosen the two spokes right next to the broken spoke and the wheel should straighten up a bit.