There are a lot of Gundam anime series, movies and OVAs, so I don't know where to start.
Can anyone tell me its order to watch it?
There are 6 different accepted universes within the Gundam Multiverse.
Each has its own characters unique to its timeline, and does not intersect with any other timelines (ex: In Gundam Wing, the events from Mobile Suit Gundam never happened).
They are listed as follows:
Universal Century Time line (Best place to start to get into it, every single Gundam series out there is rooted in characters/gundam from this time line). In order they are:
After Colony Timeline This is probably where most people are introduced into the Gundam world. A group of space colony scientists build Gundams piloted by young boys in hopes of weakening Earths oppressive Government.
Cosmic Era Timeline If the Universal Century seems to "old" for your tastes, this is a great place to jump into Gundam.
After War Timeline Welcome to your post apocalyptic Gundam universe. Basically the series take place after a giant colony is dropped on Earth, wiping out almos the entire population.
Future Century Timeline Instead of giant wars and colonies fighting one another, rulers are decided by a Gundam tournament that occurs every 4 years.
Anno Domini Timeline Several factions are constantly vying for power when a mysterious group known as Celestial Beings creates Gundams that are years beyond an current MS technology.
That concludes our journey through the very bumpy Gundam multiverse lane. I left out a few movies and tv specials, but I think I captured the bigger picture here. Now go forth, and become Gundam maesters!
The above was found here.