Head to Dumbledore's Office and you will see a girl in front of it . After the conversation you will learn that one of the students has taken a present she got from her mom. Go to the guy  showed by the girl.
He won't like the idea of giving the item back and a duel will begin . Once you've dealt with him, return to the girl  and give her back the recovered present. Afterwards, enter the office.
A conversation with Dumbledore will begin . Once it's over, follow Hermione .
Follow her all the time , until you reach the Transfiguration Courtyard, where you will meet with professor McGonagall .
Once the conversation is over, a Ravenclaw student  will ask you to show her the Levicorpus spell. In order to perform it, you must move the mouse from left to right. After the training, you will duel with a young club member, after which you can continue the tournament. You just have to approach the nearby wooden board .
Afterwards, whether you did fight or not, go to the Quidditch Gate , where you will prepare for the meeting. After the cutscene, you will have to play a match .