For this your android application must have uploaded into the android market. when you upload it on the android market then use the following code to open the market with your android application.
Intent intent = new Intent(Intent.ACTION_VIEW,Uri.parse("market://details?id=<packagename>"));
startActivity(intent);
If you want it to download and install from your own server then use the following code
Intent intent = new Intent(Intent.ACTION_VIEW,Uri.parse("http://www.example.com/sample/test.apk"));
startActivity(intent);