It's definitely a good idea to make a birth plan, even if it's not something you explicitly wrote down on paper. You need to have a general idea of how you want your labor and delivery process to go so that there will be some expectations and the nurses and doctor know what you want. If you seriously sit down and write down everything you want, there are several things that you will come up with. But there are just a few major things you'll want to tell the nurses and doctor while you're being emitted for delivery.
If you don't already have a planned C-section and no emergencies arise during the beginning of labor, you will have the option of vaginal delivery or C-section. However, unless you are dead set on having a C-section or something comes up during labor or delivery, the doctor will recommend having a vaginal delivery. The choice is still available. You can also choose to have a water-birth as well since a lot of hospitals are now integrating delivery rooms with tubs.

You'll need to decide who will want to be in the room with you during your delivery. Hospitals have specifications for how many people you may have in the room with you, and the specifications differ if you are having a C-section. The hospital I went to allowed three people in the delivery room with me, and only one person if I had to have an emergency C-section. I had my husband, my mom, and my mother-in-law in the room with me. Of course, who you have in the room with you is completely up to you; you're the one that's all exposed on the bed after all.
During labor, you have the option to get pain medication to help with the pain of contractions. Lots of women know that they will get the epidural way before they go into labor, others say that they will try going without medication, and some are determined to not get any medication at all. It's hard to tell what the labor pain will be like, even if you've given birth before, because every pregnancy is different. The most common pain medication for labor is obviously the epidural, but there are several other options so ask your doctor for other medications that they offer.

If you decide to breastfeed, make sure the nurses know so that they can help you with latching the baby onto your breast and not to give the baby a bottle of formula or a pacifier. In many cases, a baby can get nipple confusion if given a pacifier or a bottle and won't latch onto the breast. You can also get a lactation consultant to help you out and answer any questions you may have.
These are some of the main things that will need to be decided before going into labor. There are, of course, many things that can be considered; for example, who will cut the umbilical cord, if you do skin-to-skin right after the baby is delivered, and some hospitals even offer to let you watch the baby be born via mirror. It's a good idea to write these things down and take it with you when you go to the hospital to make things easier, but you need to at least have these decisions made, even if you don't write them down.