Indeed, it appears that your getJSON query is executed when the page loads. At that time, the user hasn't input anything yet, so it is executed too early.
You need to add an event listener which will detect user input, and then perform the AJAX call to the reddit API.
Assuming your user inputs his keywords in a text area, you could use .change()
.
You can fine more informations here : http://api.jquery.com/category/events/ or here http://www.w3schools.com/jquery/event_change.asp
Example in your case : http://jsfiddle.net/2ECG6/1/