1) To max out your reputation level you will first have to buy the Undercity tabard from Captain Donald Adams and then run dungeons repeatedly while wearing the tabard. However, if you have an undead character (class trials included), you can skip to step 2.
Donald Adams is present in two locations.
First is old Undercity (you can travel to the old version of Tirisfal Glades and back by talking to an NPC marked on your map with a chat bubble located north-east of ruins of Lordaeron).
Second place where you can find the quartermaster is on top of the gates of Orgrimmar but only if you have progressed the story far enough.
With the tabard equipped Any dungeon can be used for the purpose of the rep grind, but generally the better dungeons to run are Botanica Normal or Magister’s Terrace Normal. Thus, this grind can be completed entirely passively and can be combined with any other instance farming to maximize the efficiency.
2) After reaching exalted status with the Undercity faction the mount can be bought for a tiny amount of gold from a vendor Zachariah Post. Undead players can buy their faction mounts regardless of their reputation status. Thus, the fastest way of obtaining the mount is creating a Class Trial Undead character of any Class, sending them around 50g and using it to buy and learn the various Wolf mounts. The class trial character can be deleted afterwards.
Likewise, Zachariah is found in two places. Old Brill;
And the Gates of Orgrimmar once again but only if the story has been progressed enough.
Extra Tips: Countless reputation buffs can be utilized to speed up your reputation gains slightly, such as the Darkmoon Faire Buff (DMF is around for one week every month), WOW Anniversary buff or other mini-holiday related buffs such as Hollow’s End one.