The most popular leveling expansion is WoD, so that probably has the most people queuing.
Unfortunately leveling is broken up into expansions now, which means the leveling players are divided, which has resulted in players being spread to thinly.
(as a side note as a new player you’re forced to do BFA)
Your server doesn’t impact this, dungeons are cross server.
With that said, if you still want a new server, Silvermoon is the largest Alliance server.
One of the realms marked “new players” might be a good place to meet other new people. I believe that the new “newbie chat” channel isn’t realm-dependent so you’ll be able to ask for help from experienced players whenever you are.
For Alliance, some of the biggest Alliance-dominated realms are Silvermoon, Ravencrest and Sylvanas (ironically!). Argent Dawn is also high population and Alliance, but it’s an RP realm which might not be of interest to you.
I’d advise you to check out the guild recruitment forums or the realm forums for a few of these realms so you can get an idea of the people and guilds on these realms and find yourself a nice home before you choose a realm.
Wherever you end up, good luck and I hope you find a great place!