What can I do?

These days, a boy said he love's me,and he wishes I be his girlfriend, and marry him in the future,but I don`t know what I can do. In fact , he just saw me one time(We just met once).When I talked(spoke) with one of my friends online, he knew me ,then he said he fell in love with me, in fact , I don`t believe this , I think it is impossible that one falls in love with another one in the first sight.
On the other side, there is one person in my heart ,and he lives in my heart from a long time. Once, he was the person who was always there for me, and I think he is one of my best friends, I never thought that I loved him or he loved me,but until he left me, that I knew I loved him very much ,and he is not a lover for me ,he is a part of me, but he left, and I am alone, I don`t know what happened ,he said that he hated me ,and he hoped we turn into strangers in the future.But I can`t forget him, and he still lives in my heart. 
What can I do?