What impression do you form of Bassanio from his meeting with Antonio (Act 1 Scene 1)?
From the first scene in the first act, one of the most obvious things we learn about Bassanio is that he is irresponsible with money and/or he is an incompetent businessman. He claims that he owes Antonio the most, in money and in friendship. He seems genuine in his praise of Antonio as a friend, but given his debts to him, one might suspect that Bassanio is just praising Antonio in order to get more money out of him. Bassanio tells Antonio that he has a guaranteed way of repaying his debts, but it will require him to borrow more money.
Antonio replies that Bassanio is honorable and that he will do what he can to help. This sentiment (and Bassanio's loyal behavior towards Antonio later in the play) suggests that Bassanio is being genuine. He is a good and loyal friend who is simply irresponsible with money.
Bassanio's plan is to win Portia's heart. If they were to marry, he would have a share of her riches and could easily repay Antonio. Antonio allows him to use his name and credit to obtain a loan in Venice. From this we also learn that Bassanio is a romantic and whimsical or even impetuous with his plan to both win over a woman and repay the debt. So, Bassanio is a romantic, an idealist, a loyal friend, and not very competent with money.