(Branch of Fate)
After a brief scene, you'll be in the point of the game where you have to choose your side (depending on the version of the game you got. You can select any if you have the SE edition of the game). Once you reached this point of the game, you can freely select what storyline you want to follow from the title screen.
For beginners and returning players, return to your original family and defend the homeland to follow the Birthright storyline. This path includes the freedom to freely accumulate EXP and gold and will allow you to train your characters faster.
Side with your adoptive family and fight with Nohr to follow the Conquest storyline. (This is generally recommended for players who finished Birthright first) Unlike Birthright, this tricky and challenging path will give you limited access to EXP and gold. This will test your skill in devising complex strategies and careful consideration of your characters' growth.
Finally, take a neutral stance and refuse to choose any side to follow the Revelations storyline. (Recommended as the last storyline to take after experiencing Birthright and Conquest)