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Your first decision is how to react to Grindelwald's plan to enslave the Muggles. If you choose to accompany him and leave Ariana in Aberforth's care, you grow power-drunk. If you choose to ask him to stay with you until Aberforth has completed his education, he becomes your live-in lover and your sister's caretaker, you remain on good terms with your brother and you get teach at Hogwarts. This is one path that leads to a quick happy ending: you become Headmaster and enjoy following the progress of your students and alumni, especially your star pupil Hermione Granger. If later you choose to invite Tom Riddle to study at Hogwarts, Grindelwald kills Riddle to protect you and spends three years in Azkaban, during which time you must interrupt your career as Hogwarts Professor, but then you get to the quick happy ending. If you choose to accompany him and take Ariana with you, your brother and Grindelwald duel, you join in on your brother's side, your sister is killed, your brother breaks your nose, and you have more choices to make.
If you don't challenge Grindelwald to a duel, he conquers the world; otherwise you win the duel. In the latter case, if earlier on you declined to invite Tom Riddle to study at Hogwarts, you get to the quick happy ending; otherwise you have more choices to make. If you give Voldemort the teaching job he asks for, he conquers the world; otherwise you have more choices to make. If you make Sirius the Potters' secret keeper, Voldemort kills Sirius and Frank Longbottom but is in turn killed by Alice Longbottom, nobody looks for him, he remains a disembodied spirit and you take the last available path to the quick happy ending. If you make Peter the Potters' secret keeper, he betrays them to their death and Sirius to Azkaban. You now have to decide where to place one-year-old Harry - with the Dursleys or with the Weasleys. The consequence of that choice doesn't become evident until the very end of the story, if at all.