Welcome back!
What we see a lot with returners is confusion, and being overwhelmed.
If you’re not overwhelmed, you really don’t know what’s going on! 
And it’s not just what’s changed. Our memories are also less than perfect. I’ve seen people say “It used to be like this,” and … it really didn’t.
There’s a kind of double-vision effect of “I know how to do this, but … I don’t know how to do this”. It’s not just about combat either. “I used to know how to get from Hyjal to Tol Barad …”
That effect is about 100% with returners from more that a few years ago.
It Goes Away.
Whether you start a new level 1, or carry on with your Cata characters, it does go away,
Just be patient with yourself. It’ll probably take a couple or three weeks before you’re at home with the game again.
Yeah, opening the Talents page, or the new Spellbook, is a Shock and Awe moment 
, but it’s really more flash than substance. You will probably end up using a cookie-cutter talent build, and just changing a few points here and there. And while classes don’t play exactly as they used to, most of them are really very close.
If you haven’t resubbed yet, you can ofc make as many low-level characters of as many classes on as many realms as you could want, and play those for free up to level 20, with no charge for game time.
That’s what I’d do. Just quest around slowly and give the memories time to percolate back.
There is now “Chromie Time.” At level 10 you can talk to Chromie in Stormwind or Orgrimmar, and she will send you to any expansion of your choice, and you can level in any expansion from 10-70. 70-80 has to be done in the latest expansion, The War Within.
But that means that you can quest and explore in any expansion on a level 10-20, or many different level 20s. Or ofc you can stick to the nostalgia of Eastern Kingdoms and Kalimdor. Those stop giving XP after level 30, but that doesn’t bother a level 20.
Or if you want to restart immediately, I promise it will all come back to you as you go.
We see returners go both ways, and the results end up similar.
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Please let me lay out some context for your longer-term decisions.
Levelling is CRAZY fast compared to how you remember it. World record levelling from 1 to 70 is under 5 hours /played. You will probably do it in 15-20. There is good and bad in this. You probably won’t get reacclimated in that time - it’s actually too fast to teach people what they need to know to be a comfortable max-level. But your more general memories will help you there. On the other hand, alts are “cheap” to level now. One of the features of this expansion is the “Warband”, which helps shate things among alts, and that’s a recognition that because it’s so easy to level multiple characters, people now have lots of alts.
It’s probably not a lot of fun to level, get back in the game, get comfortable again … and then be stuck at 70 in the last expansion, where few are playing.
As long as you add game time, you can play to level 70, but when you’re all caught up, it’s going to start to feel a bit barren. Once you are at that stage, you will want to buy the new expansion The War Within to get to where everyone else is.
Everything up to 70 is free, covered just by paying for game time. So is Classic, Classic Cataclysm, Season of Discovery. Game time alone covers them all. But to get from 70-80, you also need to buy the latest expansion.
Cross-Realm and Cross-faction will be confusing at first. Blizzard are moving towards making realms not matter. You will see lots of people around from other servers. We are all mixed together now in the world in instanced zones. You can party with people from other realms. You won’t be doing much of that as a Free Trial/Starter Account up to level 20, but you will see the effects. Guilds are also cross-realm and cross-faction.
If you are doing the Level 20 thing for a while, I strongly suggest you follow the default path and/or ask Chromie to send you to the Dragon Isles - Dragonflight expansion first at level 10. There you will pretty quickly learn Dragonriding/Skyriding, a new and faster way to fly. A lot of people don’t like it in the first hour or so, but give it a little time and you will find it amazing. If you are one of the few that doesn’t, regular flying will be available a bit later.
Resources
You will want to bookmark
wowhead.com
to look up information about maps, quests, items, everything. They also have guides for most common things.
icy-veins.com
has a very clear and robust set of pages on how to build and play each spec
curseforge.com
where you will get your addons. Remember addons? 
and if it’s in WoW, there are like eleventy-seven videos on YouTube about it!
Do please post again if you want to know anything else, or need any help!