How do I find out what kind of furnace filter I need? We moved into a condo and there is no filter, Help?

Buy the cheapest ones. They were made for the evaporator of your air conditioner. They allow the proper CFM ( cubic feet per minute) of air flow to pass threw the filter and yet prevent hair ,dust, objects to pass threw and block the evaporator coil. The thicker filters were made for you but your air flow might be compromised and a higher electricity bill will be evident. Remember : The heating and A/C mechanics of the system is made to heat and cool. Its not a air purifier machine.