The official Ubuntu documentation is an excellent place to start learning terminal commands. Located in the Community Help Wiki section of the site, the UsingTheTerminal page covers Ubuntu terminal commands from the very beginning, teaching novices how to open the terminal window, how to enter simple commands and how to compound commands to execute multiple steps per entry.
The Ubuntu documentation is especially helpful in that it covers the usage of terminal commands in all variations of Ubuntu, including: Unity, GNOME, Xfce, KDE and LXDE. If the user is unsure which version of Ubuntu he has installed, this section helps to pinpoint that version and open the terminal window.
Once the user has become familiar with accessing the terminal window, he can move on to other resources that teach the usage of terminal commands, including BeginLinux and ss64.com. The former has a list of simple one-line commands that are used to carry out simple tasks such as changing directories, copying, making directories or accessing the calendar. The latter is an extensive, A-to-Z guide of all possible commands that are executed through the terminal, not only in Ubuntu, but in all variations of Linux. Usage of this latter option is advised only for advanced users of Ubuntu.