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
I fell first into the plastic crack then decided to check out Gundam to see what the series was about. I had just picked out gunpla I thought looked cool to start out like Zaku.
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!
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
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.)
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.
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.
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.
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.