It can be easily done providing all wires are present at the switch location to cater for a timer. The timer can bought at home depot or any electrical supply house; just make sure you ask for one that will fit in the switch box that you are installing the timer. It would be a good idea to take a photo of the switch location and show whomever is helping you in the supply house to ensure you get the right timer.
If you are not familiar with electrical installations, I would recommend you have an electrician install the timer. It can be complicated if there are multiple wires in the switch box and most timers have four or five wires to be connected depending on if the lights you want to control are switched from one location or two locations. Hope this helps.