Tuna occupies two polar opposite ends of the culinary spectrum: there is the well done fish from a metal can that is quite affordable and makes for a damn fine sandwich, and there is the seared-to-perfection ruby-colored tuna steak, which is quite a bit pricier but boy oh boy is it worth every last penny. That said, if you're buying yourself a tuna steak, you absolutely, positively, cannot mess up cooking it. That's practically a sin. Eating overcooked tuna is going to make you so sad, and you don't want that to happen.
What to Buy
Before you purchase your tuna, you need to learn how to buy it. All tuna is frozen at sea right after it's caught, so unless you live by the sea and have a special tuna guy, you're never going to see "fresh" tuna, just thawed. If it's possible for you to buy high quality frozen tuna, do that. If not, buy yours at a fishmonger or in a supermarket with a seafood counter, because those stores turn over product quickly, meaning your fish will be on the fresher side.
Ideally you'd like to shop at a place displaying an entire tuna loin on ice where they will cut steaks to order. If all that's offered are pre-cut steaks, look for ones that are vividly colored, moist, and nearly translucent, which will indicate they've been freshly butchered. Steaks that have been sitting around for a while will have begun to oxidize, making them darker and more matte. If the meat looks like it's flaking, do not buy it; that's called gapping and it's a telltale sign the tuna is old. Most importantly: check the odor. If it has a strong, fishy smell, the tuna is rotten.
Fresh, safe-to-eat tuna should have a mild odor of the ocean or smell like nothing at all.
Before You Grill
Grilling the tuna is simple, provided that the conditions are perfect. Since tuna cooks quickly, the fire needs to be extremely hot so it can create a thick sear quickly so the insides won't overcook. You need to be at full attention during the entire cooking process. Since tuna cooks quickly and tastes best if served immediately, make sure that everything else you'll be serving is already cooked and ready to go.
How to Grill Tuna
Start up your charcoal or gas grill, and allow it to heat up as long as it needs to til it's lightning hot. While that's happening, take your tuna out of the refrigerator and put it on a plate to come to room temperature. If you used a marinade, use paper towels to dry off the tuna steak completely.
If you didn't marinate it, just blot each side to dry and season with salt and pepper, and maybe a sprinkle of spices if you desire them.
Put a few tablespoons of a high-heat oil, like vegetable or canola, into a bowl, then add a wadded up paper towel to soak the oil up. Grab the paper towel with tongs and rub all over the grill's grates, then set aside. Put the tuna steaks on the grill and leave them alone without touching for at least two minutes, then peek at the bottom to see if it's charred to your liking.
Cook until you're happy with it, then flip it over to sear the other side for another minute or two. For a medium rare tuna steak, estimate around three minutes a side, but again, you need to keep an eye on it. When it's almost cooked to the way you like it, remove from the heat and plate immediately. Let it rest for 3-5 minutes, then enjoy your restaurant-caliber tuna.