How to set custom header in Volley Request

It looks like you override public Map<String, String> getHeaders(), defined in Request, to return your desired HTTP headers.

If what you need is to post data instead of adding the info in the url.

public Request post(String url, String username, String password, 
      Listener listener, ErrorListener errorListener) {
  JSONObject params = new JSONObject();
  params.put("user", username);
  params.put("pass", password);
  Request req = new Request(
     Method.POST,
     url,
     params.toString(),
     listener,
     errorListener
  );

  return req;
}

If what you want to do is edit the headers in the request this is what you want to do:

// could be any class that implements Map
Map<String, String> mHeaders = new ArrayMap<String, String>();
mHeaders.put("user", USER);
mHeaders.put("pass", PASSWORD);
Request req = new Request(url, postBody, listener, errorListener) {
  public Map<String, String> getHeaders() {
    return mHeaders;
  }
}

The accepted answer with getParams() is for setting POST body data, but the question in the title asked how to set HTTP headers like User-Agent. As CommonsWare said, you override getHeaders(). Here's some sample code which sets the User-Agent to 'Nintendo Gameboy' and Accept-Language to 'fr':

public void requestWithSomeHttpHeaders() {
    RequestQueue queue = Volley.newRequestQueue(this);
    String url = "http://www.somewebsite.com";
    StringRequest getRequest = new StringRequest(Request.Method.GET, url, 
        new Response.Listener<String>() 
        {
            @Override
            public void onResponse(String response) {
                // response
                Log.d("Response", response);
            }
        }, 
        new Response.ErrorListener() 
        {
            @Override
            public void onErrorResponse(VolleyError error) {
                // TODO Auto-generated method stub
                Log.d("ERROR","error => "+error.toString());
            }
        }
    ) {     
        @Override
        public Map<String, String> getHeaders() throws AuthFailureError { 
                Map<String, String>  params = new HashMap<String, String>();  
                params.put("User-Agent", "Nintendo Gameboy");  
                params.put("Accept-Language", "fr");

                return params;  
        }
    };
    queue.add(getRequest);

}