This is what I did on Ubuntu 18.04:
$ sudo apt install mongodb
$ mongo
> show dbs
> use admin
> db.createUser({ user: "root", pwd: "rootpw", roles: [ "root" ] }) // root user can do anything
> use lefa
> db.lefa.save( {name:"test"} )
> db.lefa.find()
> show dbs
> db.createUser({ user: "lefa", pwd: "lefapw", roles: [ { role: "dbOwner", db: "lefa" } ] }) // admin of a db
> exit
$ sudo vim /etc/mongodb.conf
auth = true
$ sudo systemctl restart mongodb
$ mongo -u "root" -p "rootpw" --authenticationDatabase "admin"
> use admin
> exit
$ mongo -u "lefa" -p "lefapw" --authenticationDatabase "lefa"
> use lefa
> exit