what kind of server you pick to play on= role playing, pvp (player vs player, or normal.
character= horde or allience (although if you plan to play with your sister or friends you will need to pick the same side). then theres what class (if your any thing like my brother you might just go to the bang bang warrior. /rolls eyes). each class has its own roll in the game and if you dont enjoy that class i sujest you try another as, not everyone likes the same. when you get higher the other aspects of the game come into play. you talk to people, make friends and plan stuff like attacks, dungeons, raids etc. it really does depend on the server and what kind of guild you pick.

wow does offer a trial so i do sujest you try it out. even if you cant maybe your sister will let you ake a character on her accout to try out?

if you decide to play just im me if you need further help.
References :

Years ago, my friend told me "life begins at 60 (the high level at the time)," and it really did. The game became significantly more strategic in that you had to play with others to defeat bosses and creatures in instances (dungeons). This required significant strategy to learn how the boss functioned and what it could do against you. You'd have to face it multiple times to come up with a way to beat it.

Further, if you PvP, the games are set up as stategy-based games – like capture the flag, hold the base or defeat the opposing army's general. This too requires significant stategy and is more complex than playing against A.I. because, we never know what our opponents – who are humans mixed with A.I. will do.
References :

The older RTS Warcraft games are what the popular WoW is based on and are great games to play.
References :
Old School