First, how many people will be at the Thanksgiving dinner? Know how many people will be eating will give you an idea of what size of turkey to get. If there will be a lot of people you may need more then one turkey.
Second, when do you need to buy the turkey? Depends on if the turkey is frozen or thawed and when you will be cooking it. If you are cooking it the day of the dinner, you will want a thawed turkey and plenty of cooking time. If you are going to cook it the day before, a frozen turkey is fine. If you buy frozen turkey, you can always put it in the freezer till you need it.
Third, when buying a turkey, consider how much of it will be left over. Sometimes, people do not like left overs. If you would like to have left over turkey, buy a bigger turkey.