In the conversation between Antonio and Salarino at the beginning of Act One, we can infer what Antonio does for a living from what is said:

Your mind is tossing on the ocean;
There, where your argosies with portly sail...

Antonio has just informed Salarino that he feels depressed, but does not know why. In his response, Salarino mentions that he might be worried about his trade ships (argosies) at sea. This obviously suggests that Antonio is a merchant and probably the owner of a fleet of ships which he uses to transport his goods.

This is further confirmed by Antonio's later statement: 

My ventures are not in one bottom trusted,
Nor to one place; nor is my whole estate
Upon the fortune of this present year:
Therefore my merchandise makes me not sad.

It is apparent that Antonio is a shrewd entrepreneur who does not place all his eggs in one basket and undertakes a variety of ventures to ensure success. He clearly does not gamble all his money on one enterprise in one particular year alone, as suggested above.

It also becomes apparent later, that because of his business successes, Antonio has built a secure reputation in Venice, a major business and trading centre, for he tells Bassanio to use his name as security to obtain a loan there.

Furthermore, Shylock's later references make it clear that Antonio is also a moneylender. He, unlike Shylock, does not charge any interest though, which makes Shylock resent him for it hampers his business as a loan shark.

He lends out money gratis and brings down
The rate of usance here with us in Venice.

Shylock means that because Antonio does not charge his borrowers any interest, he brings down the lending rate for others, such as him. It is clear that Antonio is a generous and fraternal philanthropist. Ironically, it is this beneficence which later puts his life at risk.