You can simply call redirect() at the end of function alertA(){...}

function alertA() {
    var name = prompt("What's your first name");
    if (name.length < 13) {
        alert("Hi Paul")
    } else {
        alert("Don't lie, your name is Paul");
    }
    redirect() // <<<<<<<----- calling next function
}

function redirect() {
    var r = confirm("Do you think Lee has learned JavaScript? If yes click    OK")
    if (r == true) {
        window.location = 'http://www.youtube.com';
    } else {
        window.location = 'http://www.google.com';
    }
}

You could also merge both functions into a single one:

function alertA() {
    var name = prompt("What's your first name");
    if (name.length < 13) {
        alert("Hi Paul")
    } else {
        alert("Don't lie, your name is Paul");
    }

    var r = confirm("Do you think Lee has learned JavaScript? If yes click    OK")
    if (r == true) {
        window.location = 'http://www.youtube.com';
    } else {
        window.location = 'http://www.google.com';
    }
}

You could use jQuery ui .dialog()

JS:

$(function() {
    $( "#dialog" ).dialog();
});

HTML:

<div id="dialog" title="Basic dialog">
    <p>This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.</p>
</div>