You picked a bad time to start.
I'd argue that this is probably a good time to start, actually! You have no prior expectations, youre gearing, not regearing, everything has changed but for the new twink, its all fresh. Perfect.
I keep hearing mixed things about f2p, veteran, and stopping at certain brackets on a sub—I’m not sure which one’s the best place to start.
That said, the thrust of Cletus post is accurate. So here's what I'll tell you
A lot of people like level 10 and 11 because of how scaling works. You get access to dungeons and you can be pretty OP for your level. There's no pvp to speak of but the barrier to entry is extremely low and the returns in pve are strong. That said, unless you just really LOVE smashing a 1-2 button AoE rotation... theres not much to do after the novelty wears off.
Level 20 is popular because its the highest you can play on a free account. Theres two options here, common (though somewhat inaccurately) called "F2P" and "Vet". F2P means someone who has never had gametime on their account. Ever. You start from scratch. Vet is short for "veteran" account, which is blizzards term for an account that used to have game time but lapsed. Most people who play 20s do so as "collectors" or to push achievement points, etc. Seeing just how far you can go for free, basically. I think conzil is well past 30k achievement points right now, so the ceiling is pretty high. PVP at this level is intermittent to non-existent but there are communities that do scheduled wargames, if thats your thing.
The other popular option is expansion capped twinks. Just... dont buy midnight. Hit level cap and depending on the gearing situation either hope you put in the raid time or buy Midnight greens off the AH once they become available. PvP at this level is usually available, given how many people dont buy expansions, but the quality is hit or miss. I had some great games on my 70s this past year but also had some really horrible "wait 2 hours for a lopsided GY farm". Youre going to have to relevel and regear every expansion though. Nobody sticks around at the lower levels.
Ultimately here's what Im going to heavily encourage. Decide what you want to do with twinking and pick a bracket/class you're going to enjoy playing, not necessarily whats most powerful right now. All that changes, as some people are figuring out for the first time. Meta chasing can be fun but its also gonna burn you out real fast.