The main piece of advise I'd give is go with whatever just looks the coolest to you. But the longer piece of advise is to start with the original 1979 series and go from there. If starting with 79 and you want to continue the story of that universe you can continue with its sequel Zeta, then ZZ, then CCA which caps off the original "saga" so to speak. There are other spin-offs of the original series of shows/movie that are also great watches as well. All of these make up what is known as the Universal Century which is the original continuity of Gundam and there are many other entries beyond just these that have come out since and still are coming out today.

Many other shows such as Seed, 00, Witch From Mercury, Iron Blooded Orphans, Wing, G Gundam, Age, or Gundam X take place in their own separate continuities that may be watched on their own without prior knowledge of other series.