There are a couple of things you need to know about how to grill a turkey. First of all there are a lot of factors that can influence how your turkey is going to turn out. Since you will be grilling indirectly with a low fire the weather will play a big role. This is especially true in the fall or winter months. The most important weather condition to watch out for is wind. Wind robs heat from outdoor cooking appliances, so watch it closely.
Whether you are going to use charcoal or gas makes a big difference. Gas will be easier. So depending on what you own, or what you prefer be ready for the variables. You will need an indirect fire that will hold a steady temperature in the 300 degrees F. to 350 degrees F. range. If the weather isn't going to cooperate I strongly recommend gas. You can control the temperature much easier.
Grills, whether gas or charcoal work by heating air that moves around foods to cook them. This can dry out your bird quickly. You need to prepare for this and take an active part in keeping the moisture in your bird. Your best strategies are to brine, inject and baste your bird to keep it moist and tender.
What you need:
First you will need a turkey. It would be best to stay away from anything over 15 pounds. 12 pounds is perfect. A bird that is too big may burn on the outside before the inside can get cooked. I also recommend that you use a brine. This will help keep the breasts moist and the skin from burning. You will also want to use one of those V-shaped roasting racks to keep the turkey from moving around too much. This rack should be sturdy because it won't have a solid surface to sit on. I also suggest an oven thermometer to monitor the grill temperature when you open the grill. You will also want a smoke source, to get some wood chips for the gas grill or chunks for the charcoal grill. Try a fruit wood like cherry or apple. You could also use oak or hickory. Also, and perhaps most importantly you need a trusty meat thermometer. A fast or instance ready type would be best.
Most importantly you need plenty of fuel. If you are using a gas grill you really need an extra, full tank on hand. This is just a good idea anyway, but when you are planning a big meal you don’t want to loose an hour while you try and get a propane tank filled. If you are using charcoal, make sure you have plenty and that you have a way of lighting additional coals for the fire outside of the grill. A charcoal chimney really is a must if you use charcoal.
You will also need time. Since you will be grilling your turkey at about the same temperature you would in an oven you will need about the same amount of time to get your turkey done. Remember that grilling isn’t as exact as oven roasting so times will vary. Make sure you can adjust for that. Remember if the weather doesn’t cooperate or you start getting short on time you can always move the turkey to the oven and finish it off there.