I know what Gundam is, but I've never delved deep into the franchise so I'm not quite sure where to start or what to watch. The ones I've been recommended, and seen, are:
08 MS Team
War in the Pocket
Thunderbolt
And I already know to avoid Gundam Wing and that one where it's set in the future and Gundams are used in a fighting tournament or something.
Also, once you have a good pool of Gundam knowledge under your belt, I highly recommend going to the GundamInfo channel (Completely official!!) and watching Gundam Build Fighters. It's the ultimate love letter to Gundam fans.