Jealousy is a major theme in Othello and it is what drives Othello to commit his heinous deed of killing Desdemona. Othello claims to be a man who "loved not wisely, but too well;of one not easily jealous..."(V.ii.347).
Whilst it is true that his misplaced trust in "honest" Iago and the "ocular proof" of Desdemona's infidelity drive this play to its tragic end, initially Othello does not show any signs of jealousy and is fully aware of Desdemona's friendship with Cassio which, until Iago starts his manipulative scheming, is not a concern for Othello because he knows that Desdemona "had eyes and chose me" (III.iii.189). Iago is all too aware of Othello's " constant, loving, noble nature” which he will use against him to incite his jealousy.
Honor and reputation are equally important to Othello and once his jealousy is aroused and he has the proof he needs - "I saw the handkerchief" (V.ii.66)- he must save Desdemona's own reputation "She turn'd to folly, and she was a whore" (V.ii.133) as well as his own because " nought I did in hate, but all in honor."(V.ii.298)
Othello's jealousy is therefore a result of circumstances and manipulation. Iago is such a strong personality and has everyone fooled - for most of the play- and his own jealousy appears to be the motivation for his destruction of Othello - and everyone else along the way "I do suspect the lusty Moor." Othello gives him no cause to suspect him but Iago will get his revenge " wife for wife"
To build this in to an essay, you will need a strong introduction. There have been many essays written on Othello's jealousy so you need to grab your reader's attention.
Start with a startling statement such as - From a man with a loving and noble nature, Othello is transformed in to a crazed, babbling and irrational monster.
Build your quotes in right from the start. You could continue with something like - The man "that was Othello" (V.ii.285) appears more like Iago than anyone could believe as his jealousy, much like Iago's jealousy, consumes his life and the life of all around him. How could a man, so noble in battle, so respected in society and so loved by his wife, be reduced by such an emotion as to render himself nothing more than "a malignant and turban'd Turk?"(V.ii.356)
This way you have introduced your main topic (jealousy) which is very important in an introductory paragraph. You can lead into the most planned part of your essay - the thesis statement. Ask yourself what the purpose of your essay is. In this instance, you are trying to convince the reader of your essay about Othello's jealousy and, presumably, how it is his undoing. You could add a general fact if it will reinforce your statement such as - Othello, and many more before him and still more after him , become so overcome and self-absorbed by their jealousy that communication fails and common sense has no place in a world of overwhelming coincidences and the schemes of others (in this case Iago).
You will be able to follow with paragraphs which show Othello's jealousy and also a lack of communication as Othello believes only one person - Iago- despite protestations by others. The part that Iago plays in reinforcing Othello's jealousy is also important.
Your conclusion will wrap everything up and basically prove your claims.