Your hospital won't turn you away if you arrive without a birth plan. It's not a requirement, but it can help you make decisions about your labor and delivery ahead of time.
If you do create a birth plan, include your preferences for the delivery. Indicate what medications -- if any -- you'd like to receive, how you want to deliver, and which people you want by your side in the delivery room. You should bring it to a prenatal visit and review it with your doctor or midwife ahead of time.