In order to write a character contrast, you need to follow the development of the characters throughout the play and ask yourself relevant questions about them.
In The Merchant of Venice these are some questions that will reveal Antonio's character: How does he view friendship? Why does he agree to Shylock's demands for Basanio's sake? How does he react when he learns that the debt -tantamount to his life- will be collected? What does he think of Shylock in particular and of the Jews in general? What is his private life like?
Several of the above questions can be asked about Basanio as well, although the answers are bound to be different. One important question to ask about both is what kind of love moves them.
You can think of others along the same lines. Once you have your answers, with close reference to the text, you will be ready to write your essay.
After analyzing the two character's personalities and actions throughout the play, one will find the following differences between Antonio and Bassanio in The Merchant of Venice. Antonio is a loyal, generous friend, who funds Bassanio's trip to Belmont. Unlike his close friend Bassanio, Antonio is a wealthy businessman with a positive reputation throughout Venice. Antonio is also melodramatic throughout the play, and his despair only worsens after he supposedly loses his ships. After learning that Shylock is within his rights to extract a pound of flesh, Antonio courageously accepts his fate. In contrast, Bassanio is a young, impulsive individual, who is deeply in love with Portia. Unlike Antonio, Bassanio is not wealthy and has squandered all of his money, which is why he asks Antonio to loan him money in the first place. Bassanio is also an upbeat, impetuous individual whose decisions add to the drama of the play. Unlike Antonio, Bassanio is a man of action, who is willing to risk everything to save his friend from Shylock's demands.