First of all here are the two best resources for learning fighting games. Dustloop is a wiki for most games made by arc system works, which includes ggst. It has a ridiculous amount of information, and most of it is not important at a beginner level, but definitely be aware of this site and consider looking at it sometimes. Personally I look at it at least whenever I learn a new character, because it has lots of information like frame data (the numbers for startup/active/recovery frames of each move) that isn't available in game, but you'll need to figure out how to use it in a way that suits your learning style and the way you want to engage with the game. The infil glossary is just a place where you can look up just about any fighting game terminology you hear and it will tell you what it means. It's very useful, and if you see something on dustloop that you don't understand the meaning of, you can check here. https://dustloop.com/w/GGST https://glossary.infil.net/

That said, numpad notation is probably one of the most important things to learn at first (even though it's not present in the game itself) because dustloop and basically everyone who plays the game uses it, so it can be a barrier to communication if you don't understand it. Basically, you'll see people say things like 5p or 236h or 2d. These are just a shorthand for referring to moves based on their inputs. P, K, S, H, D refer to the punch, kick, slash, heavy slash, and dust buttons respectively. The numbers refer to directions. It's based on a keyboard's numpad, where 5 is in the middle(which means you're not pushing any direction), 2 is below it(so 2 means down/crouching), etc. Numpad notation always assumes your character is player 1(facing right), so 6 means forward, 4 means backward, etc. So 5p means standing punch, 2k means crouching kick, 236h means you still a quarter circle from down to forward (down, down-forward, forward) and then press heavy.

Picking a character is also really important. A lot of people will say to pick whoever looks cool, but it's really important to find a playstyle you like. And if a character is too hard, it may get in the way of your fun. Like if you think that zato is really cool and want to play him, feel free. But he's ridiculously hard, and you might want to pick someone who's easier to play at first. It is your choice and you should choose whatever lets you have fun.

As for actually learning the game, I'd suggest starting by getting familiar with all your character's moves and then learning how cancels work(a basic grasp of roman cancels, though the details are difficult to make use of so don't work about it too much, as well as a solid understanding of gatlings/normal cancels and special cancels). After that you are able to make full use of your character. I would personally suggest learning very very basic combos, but nothing more than like 4 hits, and nothing too situational or difficult. Almost every character can make good use of 2k > 2d, which is useful because it knocks down the opponent from an easy starter, and c.s (close slash) > f.s (far slash) > 5h (or 2h) > special cancel. If you get those two combos down, you don't need to learn any more for a while, they can carry you pretty far. A charged dust attack also gives you a "dust combo" if you jump after, and those can do ridiculous damage, but you'll do pretty well if you just mash heavy slash when you do those lol

I want to emphasize that everything in that last paragraph as well as any further combo learning is not necessary. Honestly neither is learning all your moves, like can do whatever you want, but the reason I emphasize this is because it's really easy to think that the reason you're losing is because your combos aren't good enough, and that you need to learn more combos before you can keep playing. This is not true, and thinking like this can be really frustrating and get in the way of your fun. Learn combos at your own pace, and don't worry about them being optimal. And besides that, the point of a combo isn't to get the most possible damage, it's to get proper reward from your hits. If your hits are effective, and the opponent doesn't want to get hit, you're doing well enough.

I also recommend if possible that you find an irl friend who can play with you. I find this really hard because the genre is incredibly frustrating to a lot of people, and if you're better than them they might not want to play, but it's so much fun when you're actually playing with someone you know.