Gundam is several universes that are unrelated except they have giant robots colored in primary colors. The main universe is called Universal Century (UC), and started with the original Mobile Suit Gundam show in 1979. It's retroactively called 0079 ("double-oh seventy nine") after its in-universe date. It's good but has significant lumps, like the haphazard animation and villain of the week fillers. I'd recommend starting with one of the 90s OVAs: War in the Pocket (0080), Stardust Memory (0083), or 08th MS Team, as they're fairly short, self contained, and have fantastic animation. Alternatively a modern UC show is Unicorn, but there's a lot of references in it you won't get unless you've seen some of the older shows. Another option is the latest Gundam series, The Witch from Mercury. It's set in a new universe unconnected from UC so there's no prior knowledge necessary.

Witch from Mercury isn't the latest show, FYI. The latest is Gundam GQuuuuuuX (yes, really, that's the name). But I wouldn't recommend it as a first entry. Despite being set in its own universe, it's supposed to be a sort of alternate version of UC and assumes viewers are already familiar with the original.