Where is the air filter on a Carrier 58STA090 Gas Furnace? How often should it be changed? Can I do this myself?

Change it every few months unless it is a special high efficiency air cleaner. If the furnace is an upflow, the filter will be in the bottom, side of the furnace near the blower compartment, it could be inside the blower compartment also. If it is a counterflow furnace the filter will be above the furnace, again you may have to open the blower compartment and look up inside the ductwork to find them.