There are a lot of Gundam shows that take place independently from the others and thus make good starting points. My favorites are Mobile Fighter G Gundam, Mobile Suit Gundam Iron Blooded Orphans, Mobile Suit Gundam 00, and Mobile Suit Gundam: The Witch From Mercury. As Endmaker said, the original Mobile Suit Gundam (sometimes called Mobile Suit Gundam 0079) is also a good place to start if you don’t mind watching something that’s kinda old and kinda long.

It’s important to note that Mobile Fighter G Gundam is very different from the others and almost feels like a saturday morning cartoon sometimes. The other shows I listed have a far more serious tone to them, including the original Mobile Suit Gundam.