Timing belt for a mitsubishi 2003 - 2003 Mitsubishi Galant

You need to get a manual. Try the Haynes manual for your car. Try autozone; generally available for around $20-$25. This is not the type of thing that can be covered here in enough detail to do you any good. You need pictures.
General procedure is to buy new timing belt. Make sure cylinder #1 is set at TDC. Disconnect negative battery cable. Remove engine covers to access timing belt and remove timing belt Make sure all components are properly aligned and loosen pensioner to remove old timing belt. Install new timing belt. Tension belt. Double check to ensure all components are aligned properly, replace timing belt covers and reconnect battery.