Dear Prathish,
Yes, you've to first install Web Dispatcher.
1. Once it is installed, then do the reverse proxy configurations.
2. People will be able to access SAP Portal over Internet and for that your web dispatcher should be accessible through Internet.
3. Once you configure your web dispatcher to act as a reverse proxy, you can achieve 3rd point.
4. Ideally, you should have a separate server for Web Dispatcher because it is generally placed in DMZ and hence should not be installed on your SAP server.
BR,
Anurag