You're reading Othello in grade nine? Interesting.
If you're using Aristotle's depiction of plot "Othello" might go something like this:
1. Initial action / exposition: When Iago is talking to Roderigo at the beginning and we find out about the Moor's clandestine marriage to Desdemona.
2. Rising action: EVERYTHING that transpires up until the plan starts to unfold and the Moor strangles Desdemona.
3. Climax: The moor strangles Desdemona and confronts her on her supposed "cuckoldery."
4. Falling action: everything that happens after Desdemona finally dies.
As for the tone of voice in the play? That's not really a fair question because his tone shifts throughout. Think of tone as the musical scales in Music. His tone is at once shrill, fervent, cantankerous, etc. The tone is tragically loaded because of the intense emotions of the Moor. There are many ways to interpret tone.....
Cheers and good luck!