As with most things these days from changing your windscreen wipers to boiling an egg, YouTube is a great place to start, but who are the go-to guys? Well, you can of course start with my channel, but I would also reccommend a subscription to:
Mokanaman - All kinds of modelling tips.
ZakuAurelius - Modelling, kit reviews, news, interviews, all kinds! A subscription must for anyone in the community. I might make an appearance from time to time in the GunplaTalk show.
Justinius Builds - Tutorials, live streams, works in progress, all sorts. Invalueable tips and an all round nice dude.
Prefer Twitch? These channels are a must:
Not the video type? Fair enough. There's a few of us hobbyists who have embued their bottomless knowledge to the vestiges of the text-and-image based internet. Here's a few.
Child of Mecha - excellent resources and a great online store to boot, seriously, some smashing shirts and merch.
Joshudarrah.com - Looking for some serious inspiration to become a mech modeller? Josh embodies it. Great works on sale too.
Sadly, most facebook groups for Gundam / Gunpla / Mecha are filled with people who are not into the modelling aspect of Gunpla. In light of this, I only recommend the following groups as they tend to be less repetitive, and have a nicer, more relevant membership.
On discord? Seek out the Gunpla Builders Association.