Great that you're getting into gundam!
As to when you can start with gunpla: whenever you want! I built my first kit before I even watched a single episode of anything
My first kit was the High Grade Gundam Age-1 Normal. Nothing special really, but fun to build. As for what I'd recommend: either a suit you like from the anime, or just one you like the design off.
and a Tip: the newer the kit, the better it (usually) is.
Mine was the HGCE Dagger L. I still haven't seen the series it came from, but it's a really nice kit. Looks nice with no need for paint or stickers (although I did end up putting a bit of paint on it eventually.)
Welcome to Gundam! Itโs never to early to start building gunpla! Just avoid looking on the P Bandai site unless you want to drain your wallet faster!
Gunpla is freedom! No need to wait until you've seen the series; just build whatever you think looks good.
If you're worried about that, though, just build stuff from the Build Fighters, Build Divers or Gundam Breaker sub-lines. Those are just meant to be model kits even within the context of their own series. I especially recommend the HGBF Ez-SR. The base kit is good, but the main attraction is the customization parts that you can use with other kits.
Just fall into the hole of plastic crack. Your first build will be an oddly fun, satisfying experience ending with a fun little plastic robot, and then you order the next five =D
You don't need to have seen any of it to get into gunpla.
That said, you should watch the 3 movies that summarize the OG Mobile Suit Gundam, then Zeta Gundam, then Char's Counter Attack, just because they're all bangers.
I built Gunpla before I watched any of the anime. Mixed and matched mecha with other leftover weaponry. I just liked how the mecha look. People tell me things like, oh, they don't have beam weapons in IBO, etc, but I don't care. It's my build.