First, don't worry.
If it is dry yeast, don't bother with a starter. There are more cells than you need there, likely. But re-hydrating is a great idea. Pitch that dry yeast into a small bowl of water half an hour before you need it. It's best to boil (microwave) the water, then chill to room temp, first.
If you have liquid yeast, it can take a couple of days to ferment out a starter depending on how old the yeast is. If you have the time, do it. If not.... don't worry!