The answer depends on what distro you are running. Some, like Ubuntu have su not activated. Some you can enter su, and the root password, and you are root. The su command allows you to switch user, it doesn't have to be to root.
For example, if it is a buntu, see this link -->https://help.ubuntu.com/community/RootSudo
You can get a lot of info in command window with 'man su' or 'man sudo' ( all commands without quotes ).
Just google 'how to su' or 'how to sudo', either should have lots of hits.