I want to access a PHP function from Ajax

Question

My doubt is the ajax URL, I want to know how to access the function of my class

Ajax with jQuery

    $.ajax({
        type:'post',
        url: "Usuario.php?apagar",
        data: {titulo: 'Ola mundo'},
        success: function( data ){
            $('#ajax').text(data);
        },
        error: function(){
            alert('Houve algum erro no ajax');
        }
    });

PHP

class usuario extends geral
{
    function apagar(){
         echo $_POST['titulo'];
    }
}

I have no idea what the URL looks like in Ajax.