Also, if you just replace the one spoke and tighten it up to where it was before it snapped, the wheel will probably be reasonable true. However, this does nto address the reason why the spoke snapped int eh first place. Most spokes break because the wheel was built with insufficient tension or not properly stress relieved, which allows the spokes to de-tension themselves as you ride. It is likely the whole wheel should be re-tensioned before you call it fixed. Also, since spoke breakage is generally a fatigue failure, there are likely other spokes on the wheel which are also close to the breaking point.

Replace the one broken spoke, bring all the spokes up to correct and even tension, and if another spoke breaks after riding for a few days or weeks then it is time to re-lace that wheel with all new spokes.