Find cells in an array and return value for adjacent cells. Multiple results.

I don't know much about excel so I'm not quite sure how to even pose this question. I have a list of clients listed by number but arranged by date. Every time the client pays an amount there is a new entry. I want to be able to type in the client number and get the amount they have paid in fees in the adjacent cell. Client numbers come up multiple times and I want the total they have paid altogether, so I need to be able to get multiple results. I really appreciate any and all help.