Well, Im on Hellscream, which is a good server, mostly UK and sweden players it seems, but its one of the most expensive EU servers, and one of the oldest. Its also full technically right now, so you occasionally get queues, but I still like it

Ideally you dont want a realm thats been subject to a massive migration or is brand new, as there will be no one to play with really. To get the best you need a realm thats been around for a little while, but no necessarily since near the beginning, with a medium sized population, so there are decent numbers of players at any times.
Perhaps try going to the wow site and looking at the forums, read through the general a bit, then look at the realm forums, and see which seem more active etc.
You also want to decide on type of realm
-PvE
This is a standard server, you only pvp in Battlegrounds, when you turn on PvP, or enter and enemy city.
-PvP
This is a PvP combat server. Has all the features of the PvE servers but certain realms are 'contested' and thus its free for all PvP in those zones. These can be fun, but nothing worse than ganked constantly while trying to do a grinding quest.
-RP
This is like a normal server, but with a roleplaying focus. You arent meant to talk about RL out of private channels, and are for 'living' a character. Can be great fun in short bursts.
-RPPvP
RP on a PvP server. Perhaps closest to 'true' World of Warcraft, as its about making a character, living that character and personality, with more faction v faction combat.
Server type is basically your own choice, based on personal preferance. I dont mind the occasional RPPvP as its a laugh sometimes, and as a world is a little more developed/involved, with people taking more time to roleplay, rather than grind/quest/instance etc, but too me thats an occasional indulgeance for fun with mates, whereas some people prefer to play like this all the time, and I can see why some find it drawing, it just gets a bit *too* over the top for me, and I play a PvE realm where I can chill with RL mates, and chat about whatever, and have fun in a game together.
OHH biggest tip for WoW. Do NOT beg for gold or harrass people for help, as you will royally P people off, and its much nicer to make friends, and instance, rather than contacts who hate you.