First look underneath near water tank and open the water drains, either caps or plugs.
Next hook up city water connection and flush out lines with kitchen or lav faucet open till you get clear water. Put some water in the fresh water tank with low water drains open and see if its draining clear or anti freeze. Remove your water filter and install new cartridge. With the anode still out open the cold water inlet valve behind your water heater.
After flushing water heater, install anode either new or old if still good, now open hot water valve behind water heater and open sink and or lav hot water to flush hot lines.
At this point you can close low water drains and sanitize your water tank and lines. There are many differant threads on How-To Sanitize.