Steps
Edit your SharePoint page and add a script editor.
Paste the below code in your script editor.
Source code
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.1/jquery.min.js"
type="text/javascript"></script>
<script type="text/javascript">
$(document).ready(function()
{
//Set the Field to Read only and change its background colour
$("input[title='FieldName']").attr("readonly","true").css('background-color','#F6F6F6');
});