Complete both quest lines at your leisure simultaneously but avoid advancing past the Main Story quest 'Alduin's Bane' (the prerequisite of The Fallen quest) until you have completed the Civil War quest that allows you take the main fortress of the opposing side in the war, namely 'Fort Greenwall or Fort Sungard'.IMPORTANT: It is best to complete the Civil War quest line, at least up until the point where you unlock the 'War Hero' achievement, before reaching the 14th Main Story quest 'The Fallen', see below.

The Main Story can be completed in a minimum of '20' hours depending on your character level, difficulty setting, your general skill, and your gaming style. During the course of the Main Story you will unlock '8' achievements for the completion of certain quests and a further '3' achievements for completing tasks associated with the tutorial aspect of the story. The Main Story has '17' mandatory quests and '2' optional quests. However if you are only planning a single power playthrough then I'd advise you hold off on the Dragon rising quest and complete as many Daedric quests as possible before returning to it. There is a choice to be made here that I would not like to dictate for you, I will simply say this: If you plan on playing multiple playthroughs, like suggested in the General Hints and Tips page, then you should complete the fourth quest as and when you encounter it, it offers new character advantages and continues the tutorial of the game. I should note here also that completing the fourth main quest 'Dragon Rising', you will make the Daedric quests, and some other side quests, more difficult later on.

Most common races in skyrim