Complete Bronze Proving Grounds, get your weapon

Of course the first thing we want to do upon hitting level 100 is to start gearing up. And we want to run heroics, which means getting a silver in proving grounds. Well, while you're at it, don't miss this quest, given in your Garrison by Lieutenant Thorn if you're Alliance, and Warmaster Zog if you're Horde. It's a good way to get your hands on a solid 610 weapon (almost as good as an heirloom at level 100) and, if you get lucky and get an epic instead of a blue, it's actually ilevel 630 and better than the heirloom weapons from Garrosh.

I did Bronze as DPS first, and found it to be pretty easy. It's basically a free weapon as good as a dungeon drop, with a chance to be as good as a heroic dungeon drop. Since you're going to want to be able to queue for heroics anyway, there's no reason not to get it done.