For the first step, you should use a Roku remote. Once you have it connected to your Wi-Fi, then you can get it to work with the app.
If you don't have a remote, and if the Roku used to be connected to a Wi-Fi router, then a trick you can use to avoid having to get a remote is to set the router to have the identical SSID and passphrase of the previous router. Then your Roku will just (re)connect to it.
