It depends what you mean by that.
- If you want to shut down all GUI instances and go down to a (root) command line prompt, just do
"sudo telinit 1"
- If you want to have your machine boot into command-line (CML), follow the same procedure as here: Lost password in CentOS - #2 by triantares
- On a running machine just hit "Ctrl+Alt+F1" to get into a terminal, log in with your usual user credentilas and go about your business.
Hit "Ctrl+Alt+F7" to get back into the GUI.