Send data with ajax, return json

I want to send a string and return json.

This works but it doesn't return json code

$.ajax({
    url: "getFeed.php",
    type: "post",
    data: myString
});

When I want it to return a json string, it gives me an error.

$.ajax({
    url: 'getFeed.php',
    type: "post",
    data: {string1: "testdata", string2: "testdata"},
    dataType: 'jsonp',
    jsonp: 'jsoncallback',
    timeout: 5000,
});

For some reason the server doesn't recieve any data.

How am I supposed to write this? (i want to send 2 strings)