Step 1: The temperature in your oven has to be accurate. You might want to test your oven by preheating it to 250° and testing it with a cooking thermometer. I highly recommend this step. It can make the difference between a moist, juicy turkey or something that resembles a wooden block.
Step 2: Your turkey needs to be safely and totally thawed and cleaned. The only safe way to thaw a frozen turkey is to place it in the refrigerator. Other methods such as running cold water over it or placing it in a microwave oven are not safe because of the chance of bacterial growth and contamination. Because of that risk, I will only give you guidelines using the refrigerator method. Most store-bought turkeys will have instructions for thawing. I recommend that you read them prior to Thanksgiving.
Step 3: Be sure to remove the neck and giblets from the inside of the turkey. When people get in a hurry, they often forget a few items. This one can easily be overlooked.
Step 4: Know the actual weight of the turkey excluding the giblets so that you can accurately calculate the cooking time of your turkey.