getJson接受用户输入

I have an application which requires us to get user input- i.e user should be able to browse through the files in his system and upload a json file- the json file's path should then be passed onto the url field in the getjson function. How would this work?

My html looks like :

<style>
.container {
    width: 500px;
    margin: 0 auto;
}
.progress_outer {
    border: 1px solid #000;
}
.progress {
    width: 20%;
    background: #DEDEDE;
    height: 20px;  
}
</style>
</head>
<body>
<div class='container'>
<p>
Select File: <input type='file' id='_file'> <input type='button'    
id='_submit' value='Upload!'>                
</p>
<div class='progress_outer'>
<div id='_progress' class='progress'></div>
</div>
</div>
<script src='upload.js'></script>

upload.js in turn refers to upload.php and then I have my getjson as follows ( the getjson method is on the html page):

 $.getJSON('upload.php', function(json){//}  

However, I feel the getjson (upload.php) is not working. I'm not sure if it is able to fetch the file path from upload.php. Can someone please help. Where am I going wrong?