No you can't screw them up, the only thing you can mostly do wrong in rpg's is putting your skills/attributes wrong, there are no attributes in WoW so no worries about that and skills or in WoW words Talents have to be placed yourself, but there are enough builds online to get a good one and even if you make a wrong one still no worries because for a couple of gold you can replace all your talents.
It all depends on how you play your character and also on equip, but you get equip by instancing and by instancing you get better playing your character and thats something you can't buy, even if you buy a uber char on Ebay you will still be a newbie at start, just make sure you have someone accompaning you on your quests and it won't be that boring.
I had a friend who was online at the same hours as me and we always teamed up, so we could always chat with eachother that helps alot.