All pizzas start with the dough. When grilling a pizza you want good dough that will hold up to being transported and turned on the grill. Roll out your dough about 1/4 inch thick and try to keep it pretty even. Pizza is a very forgiving dish so you don't need to break out the calipers for this one. Just keep the dough uniform and well floured.
When it comes to the grill, keep it clean and well oiled. You need the pizza dough to slide on the cooking grate. If it sticks you'll be out some dough. What you want to do is take the rolled dough rounds and lightly grill them. You can grill them on one side or both. If you just grill one side put your toppings on this side before you return it to the grill. Either way, with the bare dough rounds lightly browned, put your toppings on and turn down the heat on your grill. If you are using charcoal you will what one side hotter than the other so you can put the topped pizzas on the cooler side. The goal here is to get the toppings heated and any cheese melted before the crust burns.
As with all grills, keep a close eye on the pizzas. With the lid down you can bake the toppings to get your pizza just right. If you are using heavy toppings, like meats or thick cut vegetables you might want to grill them up a little first before you cut them up and put them on the pizza. While the grill is a great place to cook pizzas the intense heat can burn through crusts pretty quickly. You don't want cold toppings on a burnt crust.
Now that you have the basics down you can enjoy the best part of pizza, the versatility. When it comes to pizza, there's no such thing as a bad topping. Another tip is to make small pizzas for gatherings and let your guest choose what they want on their pizza. The grill is a fast way to prepare pizzas so you can cook a lot of them in a short time.