You can find the answer to this question very early on in the book. It comes right at the beginning of Chapter 3.
The reason that the two of them are drawn together as friends is that they both have a love for books. This is not something that we know about Jeffrey (who later comes to be called Maniac) at this point in the book. But we find it out when he sees what Amanda has.
Jeffrey notices Amanda immediately because she is the only kid who is going to school carrying a suitcase. He thinks she is a runaway like him and so he talks to her. But then he finds out that she isn’t carrying the suitcase because she’s running away. She’s carrying it because it contains her books. When he saw the books
Jeffrey fell to his knees. He and Amanda and the suitcase were like a rock in a stream; the school-goers just flowed to the left and right around them.
From this, the friendship starts. Amanda tells Jeffrey about her family. Jeffrey begs to borrow a book from Amanda. That is the start of the friendship.