All world bosses and some dungeons are meant to be done in a group. It doesn't matter what your level is because there is level scaling, but you do usually need at least two other players to help. The only part that is (was? I'm not sure but it was when I first started playing) is Cyrodill because it's a PvP zone you needed to be level 10 to access it. Not all of Tamriel is in ESO yet because those will either be sold as dlc or 'chapters' (read: expansions) or the servers might get shutdown first. Long story short, when you make a new character the game will drop you into whatever expansion beginning you have and from there you can just run off in a direction--that's part of the reason why I have so many hours in one character and I'm not even done with one faction quest line, because when I get bored with a zone I'll leave and go do guild quests or just go to a new zone for a change of scenery.