Season it with salt and pepper and let it sit for a few minutes. Get the grill good and hot, hood up, grill for 4–5 minutes on one side. Flip it, turn the heat down, and stick a constant-read meat thermometer into it, and grill it until it reaches 130–135 degrees. Take it off the grill and let it sit for 5 minutes. Perfect medium rare.
If you’ve got experience, you can tell the doneness by the firmness of the meat, as @essieness suggests, but that’s not something I’d recommend for a beginner. Also, if you have experience with lots of steaks and that particular grill, you can probably estimate how long it will take without needing a meat thermometer. But the meat thermometer is the only way to be sure.
(You can also do this indoors—season the steak, get a cast iron pan good and hot, and cook the steak for 5 minutes on that side. You will get smoke. If you don’t get smoke, you’re doing it wrong. Then flip it and put the whole pan into a 400–450 degree oven, with the meat thermometer as above, until it reaches 130–135. Put the steak on a plate and let it sit for 5 minutes before serving.