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You need to open a terminal emulator if you're in a GUI environment. Which terminal emulator you have handy depends on what GUI environment you're using.

If you're using KDE, you'll have Konsole available to you. If you're using Gnome, it'll be gterm. Either way, there's probably an icon on your taskbar / panel / whatever, at the bottom of the screen, that looks like a computer monitor with a black screen and maybe a command prompt or a seashell attached to it. That icon will open a terminal emulator.