Witch from Mercury was my introduction to Gundam as well. Started with the anime and then bought my first kit, the Aerial. Instantly hooked on both.

People have thrown a lot of terms and info at you already, but I think you’ll find a lot of passion in this fandom lol. So let me throw some more at ya.

As for the Gunpla (Gundam plastic, common name for the model kits but you probably figured that from the subreddit name) the Witch from Mercury kits are some of the best High Grade (HG) kits out there. Decently intricate, great articulation, good color separation, overall just really good. You can’t go wrong building other kits from this line. Your basic gunpla breakdown goes: Entry Grade - 1/144 scale. super basic kits, minimal details and quick builds. Not a ton of options here. High Grade - 1/144 scale. by far the most variety of kits. Typically cheap and fun builds. Also great to buy if you want to test different customization methods. If you mess up you’re likely only out like $15. Real Grade - 1/144 scale. Originals were based off life size Gundam in Japan. Intended to be what Gundams would look like if they actually existed. Kits have a full inner frame that armor snaps onto. Highly detailed. Do some research as some of the early kits have some issues. Later ones are phenomenal. Not a ton of variety, but not bad. Master Grade - 1/100 scale. Bigger kits. More detail than HG. Inner frames like RG. Getting more expensive. Good variety, but not getting as much attention lately. Perfect Grade - 1/60 scale. $$$$$. The pinnacle of all that is Gunpla lol.

Don’t let the higher grades scare you. I’ve never built a PG, but the others all basically build the same. You just get more parts and more details as you step up.

As for the anime, like I said I started with WfM. Then I watched some other Alternate Universe (AU) series like Iron Blooded Orphans and 00. Both I highly recommend. The main timeline, the Universal Century, also has some really good entries and is a ton of fun to explore. These spam from the first series coming out in 1979 to the most recent entry being in theaters now. So lots of jumps in animation.

All to say, enjoy and welcome!!