Is this a good essay about how opposite themes are portrayed in Romeo and Juliet?First of all, Romeo, and Juliet is the most famous love story in...

There are plenty of good ideas in this essay, but it definitely needs some organization and to be broken up into paragraphs. Also, unless your teacher indicates you can use the first person I in your essay, eliminate it. The first three sentences seem fine for an introduction though I've made a few edits:

First of all, Romeo and Juliet is possibly the most famous love story in English literature. Love is naturally the play’s dominant theme. The play focuses on the intense love that springs up between Romeo and Juliet despite the bloody feud between their families.

Following these sentences you need another introductory type sentence about hate. It's always good to insert actual quotes from the play to back up your points and there are numerous possibilities—maybe use the quote about "hell" involving Tybalt and Benvolio. Then you need a strong thesis statement indicating that the point of your essay is about how the opposites of love and hate fill the play.

You probably need at least two body paragraphs for your essay. You have the choice of two directions in this. Each paragraph could deal with a different topic. For example, the first body paragraph could be about the secret love between Romeo and Juliet. The second paragraph would then be about the hatred between the families. Make sure to use specific scenes from the play to provide evidence. Direct quotes are important as well. Or your body paragraphs could combine the two themes and juxtapose an example of love with an example of hate.

So, in your first body paragraph make sure to provide a topic sentence which tells what the paragraph will be about. I've lifted a sentence from your essay and provided editing to give an example of a possible topic sentence for the paragraph on hate:

The hate between the Montagues and Capulets is deadly, long standing and it seems neither side is willing to put it to an end.

Then give examples of this hate (the fight in Act I, Scene 1 or when Tybalt becomes enraged at seeing Romeo at the party). Also make sure you provide your opinion about those examples. For instance, you might indicate that the feud is immature or childish (especially the "thumb biting" episode in Act I, Scene 1). Provide at least two examples of hatred in the play and your opinion about those examples. 

Do basically the same thing in your second paragraph. Write a good topic sentence, provide examples of love in the play (Romeo and Juliet first meeting, the balcony scene, their wedding) and then give your opinion about those examples, such as "Juliet's beauty overwhelms Romeo and he seems to instantly forget Rosaline as he compares Juliet to a bright light and a jewel in Act III, Scene 5 when he first sees her."

For your conclusion, I have edited your last two sentences. Two sentences, however, do not make a good conclusion, so you'll need more:

In conclusion, there are two very strong emotions in the play, love and hate. The play portrays the chaos and passion of being in love and how hatred can make that love difficult.

Your essay should be four or five paragraphs and make sure that each of your paragraphs has 5-8 sentences or more. Don't forget the thesis statement should be the last sentence in the introduction and that each body paragraph needs a good topic sentence.