I'm the only kid on this block...
Jk, Welcome aboard dude!
To answer your questions, you have two options, either Icecrown or Lordaeron;
Icecrown is a super old realm (two old realms merged into one) with a huge population, 12k online everyday 12 hours a day, it's a more PvP focused realm, it has PvE of course but it's been out for such a long time that everyone is like geared to the teeth with 4-5 characters, the Rates are x7 for XP and x5 for the rest.
Lordaeron on the other hand is Warmane's Pride & Glory, it's there baby, Crafted meticulously to deliver the must complete experience, and it does. Though it's a x1 Blizz-like Realm, which means you gotta grind for that level 80, the good thing about Lordaeron is that it's a progression Realm, meaning they release content Tier per Tier and Season by Season. Unlike Icecrown, Lordaeron is a more of a PvE focused realm.
Both have great communities as far as PvE community is concerned, so the only realm differences are XP Rates; x7 for Icecrown and x1 for Lordaeron, and the Content available; All of it for Icecrown and Ulduar (ToC soon-ish) for Lordaeron.
That'd be it for the Wotlk Realms, i wish you a good journey in here, and have fun ;)