The two choices that must match
Your characters must share a faction and ruleset to group for quests, dungeons and raids. Alliance and Horde cannot form a party. Being Battle.net friends does not bypass either restriction.
Write the agreed choices in your group chat rather than saying “the same server”. Forever starts with a ruleset choice. Different rulesets are not channels that you can freely switch between.
Choose Normal, PvP or roleplay together
For a group that includes new players, I would choose Normal unless everyone actively wants open-world fighting. On PvP, enemy players can interrupt a questing evening. That is part of the experience, and it cannot simply be switched off for the least experienced member.
Roleplay suits a group interested in names and character identity. Roleplay-PvP adds open-world conflict. Hardcore is not available at launch, so do not make it the basis of your first group session.
Choose a meeting point, not a forced march
Sharing a starting area is the simplest option. Humans begin together; dwarves and gnomes also share their early route, as do orcs and trolls. If different races matter more, finish your local opening tasks first and arrange a later meeting in an accessible settlement.
The Deeprun Tram connects Stormwind and Ironforge for the Alliance. A tauren joining friends in Durotar has a longer journey. Plan that as a proper travel segment rather than sending a fresh character alone through unfamiliar enemies.
Compare your actual quest objectives
Before clearing a camp, briefly compare active objectives. If someone cannot accept a quest, check their level and prerequisite quests first. A shared quest offer does not remove its requirements.
Do not rush off the moment your own objective is complete. Ask whether everyone has what they need. Staying together saves more time than a fast personal turn-in followed by half the party having to backtrack.
You do not need a fixed composition to level
Pick classes you also enjoy on your own. Ordinary questing does not require someone to stay in a role they dislike. Decide who tanks, heals and deals damage when you plan your first dungeon.
You can coordinate professions: one friend makes bags while another gathers herbs or brews potions. Agree what happens to materials first. A shared supply works best when everyone knows what is a gift, what is for sale and what is needed for personal training.
If you cannot see each other
Check region, faction, ruleset and your actual location. Matching place names are not always enough in buildings with several floors. Try the party invitation and compare quest progress.
Preferred Online Language is under Options → System → Languages and Social. It is a preference for encounters, not a way around faction or ruleset restrictions. End the session by agreeing a resting place and your next objective, so the following evening starts without another search.