This is completely up to you. I have seen very detailed birth plans with very specific instructions for any and all circumstances and those that have the bare minimum. From what I've seen I would discourage the overly detailed birth plan and if you are going to take the time to write one be thoughtful and do not write one that would just cover standard practice.
So what should you include?
If you are not wanting any medication, write that you do not want to be asked about pain medication and that you will let them know if you need something. Know that they have to tell you your options, so allow them to tell you your options, but then you can nicely say, "Thank you for letting me know, I will tell you if you need anything."
If you do not want an episiotomy, make that clear.
Feel free to list your room preferences such as lighting, music, temperature, etc.
If you have strong feelings on induction or breaking water include them in your birth plan.
Write down any preferences for what you would like to do during labor and pushing.
Include anything for newborn care that is different than what is standard practice, such as not wanting them to have they eye ointment or the vitamin K shot.
Write if you are wanting them to breastfeed only or desiring formula.
Write if you want your baby boy circumcised or not.
These are the basic things you can include. As with other things be kind and gracious in how you approach it and know that things can go differently than expected.
Here and here you can find two types of check off birth plans. If you have any good resources for these please share them below.
What do you put on your birth plans?