Go to Uplay from the main menu and register. You can then earn Uplay points by doing these tasks:

10 Uplay points - Get the "Mystery Guest" achievement by completing Sequence 2.
20 Uplay points - Get the "Tea Is For Englishmen" achievement by completing Sequence 6.
30 Uplay points - Get "The Sum Of Truth" achievement by completing Sequence 12.
40 Uplay points - Get the "Abstergo Entertainment" achievement by reaching Level 20 in Multiplayer mode.

Then once you get the points, you can spend them on the following items:

Assassin's Creed 3 Theme - Purchase for 10 Uplay points.
Multiplayer Profile Items and Pouch Upgrade - Purchase for 20 Uplay points.
Ezio's Oufit - Purchase for 30 Uplay points.
Multiplayer Profile Items and Night Stalker Outfit - Purchase for 40 Uplay points.