That's a big can o' worms...
The original Mobile Suit Gundam TV series spawned off the entire franchise, including sequels, spin-offs, movies, OVAs, etc.
Basically, you have the "classic Gundam" that takes place within the Universal Century calendar. Many of them are made with original director Tomino, although lately most UC shows are OVAs/one-off CG productions/video games done without his participation that just try to stuff more useless junk into the One Year War period for the sake of whoring more merchandise out of this cow that has been milked dry, and then some. All UC Gundam shows are somewhat related, since they take place in the same universe and are only a few years or decades apart.
On the other hand, you have the "alternate universe Gundam" that includes the likes of G, Wing, X, Seed, and its spawn Seed Destiny. These are mostly made for kids and are often pale imitations of the original, although I am sure a lot of kids will disagree vehemently about this statement. These shows are stand-alone series that do not make reference to other Gundam shows (except for Seed Destiny, which is a direct sequel of Seed) so prior knowledge of other Gundam shows are not required.
Gundam 0083 is a good OVA series to get into Gundam with. So is the Mobile Suit Gundam movie trilogy, which is basically the original TV series condensed.