I look for a good fresh cut of top sirloin steak.  Don't be afraid to talk to the butcher.  Ask him if he can cut a fresh steak.  Don't settle for one that is brown on parts of it due to air exposure and time.  It should be bright red with good marbling throughout, that marbling is delicious fat.  Trim back any excess fat as this will only lead to the dreaded flare ups. Take the meat out of the refrigerator 1/2 hour before to let it come up to room temperature.

Prep the grill.  Get it really hot with the lid closed for about 15 minutes.  We are going to be cooking with the lid open, but we want the grill as hot as possible at first to get a great sear on that top sirloin.

I can't tell you exactly how long to cook the steak for, because it depends.  Depends how hot your grill gets, how thick the steak is, and how well you like it done, bone in or boneless.  I look for browning on the edges.  Since I like medium rare to medium, I wait until it starts to show brown 1/4 to 1/2 way up the side of the steak before flipping the steak.  For medium rare on a 1 inch top sirloin boneless steak, total cooking time should be about 8 to 10 minutes.  For a  2 inch boneless top sirloin total cooking time of about 18 minutes will give medium rare results.  Ideally, you should be using a great instant-read meat thermometer.  For medium-rare, cook the steak until the internal temperature is 135 degrees, then remove and allow the top sirloin steak to rest for 5 minutes.

Use a good pair of tongs to flip the steak about half way through the cooking time.

Once it is done, let it rest on the plate for 5 minutes before you cut into it.  This lets the juices redistribute and keeps them from running out all over your plate.