To start with its all about the cut of meat.
My personal favourite it the Rib Eye, but Porterhouse and TBones are good as well.
Go with at least a AA cut, look for good marbling throughout the meat, that is the fat that runs through the meat, better marbling means better flavour, good moisture, and tender.
Also pre season your steaks at least 2 hours before you cook and let them come to room temperature before tossing them on the grill. I like to use Garlic Powder, Onion Powder, Fresh Ground Pepper, and Seasoning salt. (The Rub) its called a rub for a reason mix up the above in equal quantity and rub it into your meat, this ensures good coverage and gets the seasoning into the steak as well. Some would say not to put on the seasoning salt as it dries out the meat, I say bullshit, it will only dry out crappy meat.
Before putting the grate on, spray it with Pam (use the hi temp grill stuff if you can get it). That helps prevent sticking. Steak isn't bad about sticking, but some of the fats and drippings may be hard to clean off if you don't pre-spray.
Use tongs to place the steaks on the grill. Do not use a fork. When you make holes in the meat, the juices run out. This makes the meat dryer and less tasty.
Some grillers like to sear the meat and turn only once. This technique is hard to master, especially if you don't have a searing grill or are using coal, and I don't think it makes for a better tasting grilled steak.
I typically only flip a steak twice, the idea being to cook it evenly to push the juices into the steak, constant flipping is a pain and I don't think it helps. I like to cook lid closed to keep in the flavours of the drippings, also if you are like me a little smoker box with some mesquite in it will do wonders for your steak.
When the coals (or heat setting) are correct, it takes about 15 minutes to cook a steak to medium. This varies from grill to grill, with the heat, and with the outside temperature. Keep a spray bottle with distilled water in it nearby for flair ups, they add a charred taste to your steak and me I don't like that, unless you are looking for a Chicago style steak. Make sure you have a fresh beer prior to starting the steaks as you should stick with them till there done, that way you can monitor temperature and flare ups, its worth the 15 minutes.
Try not to knife check too early, or the juices do leak out. Once you've cooked a few steaks, you'll probably be able to eyeball them and tell when they are cooked to the desired level of doneness. When you are done with the steak do not leave it on the grill on the top rack or with the heat low, this will overcook your steak and dry it out. I like to take my steaks from grill to serving plate, but not on your plate yet. Let them stand for about three to five minutes to let the juices settle in the meat (makes for a juicier, more flavourful steak in my opinion)
Enjoy, I have been told on many occasions that I grill the best steak people have ever had. I do some mighty fine ribs too if your interested in how to do that :ss