He goes into their room while she is sleeping and then she wakes up and after she does he smothers her
Act 5, Scene 2
Iago had lied to Othello and convinced him that Desdemona was secretly having sex with Cassio.
He asphyxiates her
Othello is a play, so you should not think of it as a book. Go and watch it, either at a theatre or on film, if you want to understand it. Learning how to understand plays by just reading the script...
Othello 's wife who was dead by him in the end of the story.