The aldor is the second of the main factions you'll encounter in the Burning Crusade. But, as I mentioned in my Scryer reputation guide, you can only choose one of these.

How do I choose Aldor?
To choose the aldor as your faction you'll have to first complete the Shattrath City introduction quest. Once you've had the grand tour of the place simply talk to Khadgar, he will allow you to choose either the scryers or aldor at this point.

Make sure you don't choose the wrong one because it is incredibly difficult, time consuming, and expensive to change your reputation later.

How do you gain reputation with the aldor

The scryers are one of two main factions in the Burning Crusade (the other is aldor). Both of these factions provide great rewards, but as they are at war with each other you can only pick one.

As you gain reputation with the scryers your reputation will go down with the aldor, and vice versa.

How do I choose Scryers?
First things first, you must choose between the aldor and scryers. You do this by talking to Khadgar after you've done the Shattrath City intro walk through quest. You can find him in the middle of Shattrath city.

How do you gain reputation with the Scryers?
There are several ways. The most straight forward is by simply doing quests for NPC's of the Scryer's faction. These are located throughout the world, and generally speaking they all give loads of experience.