you are logged in as root. You need to start the Junos cli via the 'cli' command and then enter the set commands in configuration mode.
this mode is accessed by entering the 'configure' command, then the needed set commands and 'commit' to activate the changes.
I will recommend you to read up on some of the Junos basics. That can be accomplished via for example the Day one book 'exploring the junos cli, second edition': https://www.juniper.net/uk/en/training/jnbooks/day-one/fundamentals-series/cli/