How to make entrecôte steaks on a grill?

Thickness of the steak is really personal preference. There is no "right" answer. The thicker it is, the easier it is to get a nice rare to medium rare in the middle with a good crust on the outside. The thinner it is, the easier it is to get the center well done without turning the outside to charcoal. I personally like my steak between 1.5 and 2 inches.

Spices are in the eye of the beholder. Personally, I'd just put salt and pepper on it. Depending on the quality of the meat, I may put some Worcestershire Sauce on it. Or maybe Soy, if it's an asian dish. Stay simple though. The flavor of the meat should shine through, so don't overly spice it.

Ideally you want a two level fire to cook steak on a charcoal grill. One side should be as hot as you can get it. Sear for 2-3 minutes per side over the hottest portion of the fire. The second level of the fire should be medium hot. Move the steak over once it's seared to finish cooking. Keep the grill lid down for the second portion. If you have a gas grill, just set one burner as hot as it will go and one on medium. Cooking over a medium heat will allow you to finish the steak without over doing the outside. Time depends entirely on how thick the steak is, temp of your grill, and how you want it done. Learn to tell "done" by feel rather than time or get a thermometer.

When the steak is done, you need to let it rest for 5-10 minutes. Otherwise, when you cut in to them, you lose a lot of flavorful juice. Resting allows the meat to retain the juice. Wrapping in tinfoil while it's resting will help it retain some of it's heat and be a bit warmer when you serve it.