you enter phone but klick on the key thing on your controller twice to enter cheats (only on single player) You would enter them into your phone. Once entered, you can then re use them by going down to the Cheats Section of your Phone and selecting the desired Cheat.
Depends on the game your playing. If the game your playing has cheats it will normally be in options but some have complicated ways to enter codes which you can find out from cheat sites like gamewinners.com or gamefaqs.com. If the game doesn't have cheats you can't enter cheats.