Turn A gundam was supposed to unify all of the Gundam Universes thats what they refer to as the "Dark History". If you want to watch some of the newer stuff like Igloo and Unicorn then you're going to have to go all the way back to Mobile Suit Gundam and progress from there. For stuff like 00, Wing, Seed you can watch those without having any previous knowledge. This is the order in which I watched Gundam.
Wing
08th MS team (it was on Toonami at night and had nothing better to watch... kick ass OVA though)
0079
0080
0083
SEED
Zeta
ZZ
Chars Counterattack
F-91
G-Saviour (Only got through 15 minutes of this)
G-Gundam
Turn A Gundam
Igloo
SEED Destiny
Gundam 00
Gundam Unicorn (This is one badass OVA for those who havent seen it)