If you're new to twinking, the best place to start is either with a F2P/Vet toon or an expansion capped 50 (IE, a 50 on an account that doesnt have Shadowlands)
Reason for this is pretty simple. Blizzard has separate pvp ques for people with xp-on and xp-off. Levelers, obviously, are in xp-on while the only people with with xp turned off are twinks. But the twinking population is quite small (among other things) so getting games that are strictly twink vs twink is very difficult and requires gobs of coordination.
But xp-on ques? Well now we're talking the whole population of the game, which means BGs pop quickly and regularly. And F2P/Vet accounts and expansion capped accounts can que for XP-on because we won't level due to our account status. This lets us maintain a twink and get easy "plug and play" games (to steal a line I really like from Bestworld)
I would very much recommend you dip your toe into twinking with a Veteran account. That is, start a separate world of warcraft account on your bnet client. Level a toon (or several toons) to 20, turn your xp off and start gearing, acquire consumables and stacks of enchant scrolls, etc and when you're ready, let the accounts subscription die. Make sure you turn your xp back on right before it ends, however. Then (and this is important) exit the game, exit the Bnet client, and go get dinner or watch a movie or something. There's an odd glitch where people who were playing when their subscription ended will end up leveling anyway. Give the system time to catch up to the status change.
Once thats done, you'll have a veteran account with a ready to go twink! Enjoy BGs.
Should you find that enjoyable and want a bit more of a challenge (and *I* think, more rewarding playstyle) you might dip your toes into pure F2P. That is, an account that never has game time. But that's a bit more involved.
Here's some helpful resources:
Starters Guides to 20s in SL - Very basic rundown of available classes, talents, abilities, initial gearing, etc.
F2P and Vet Armory - A collection of various toons of all specs so you can see what gearing looks like
SL Info dump - a hodge podge collection of various things we've found/learned. Check dates on these posts, a lot of this is no longer relevant but theres some good stuff in here.