Check the Oil
You can check the engine and fuel level and if need be top up either the fuel or oil. You may also change the oil if the is a manufacturer recommended oil in the operator's manual. You can also check the tires pressure but keep it at the value marked on the tires or if there isn't one keep it between 15 and 25 PSI.
Check the water inlet screen or the in line screen and flush them with water if they are dirty. If the inlet screen is damaged replace the screen and rubber washer. If the in line screen is damaged replace the screen and O-ring.
Inspect the Hoses
Inspect the high pressure hose and replace if you see any leaks, cuts, abrasions or bulging of cover or if the couplings are damaged or move. New hoses will have to exceed the maximum pressure of your washer. Also examine the cleaning detergent tube and filter. You will need to clean these if they are dirty or clogged. Examine the tube for leaks or tears and make sure it fits lightly on barbed fitting. Replace the tube or siphon/filter if either are damaged. Replace the o-rings if any connectors leak and clean nozzle orifices if clogged. Also rinse out the garden hose.
Make sure to flush the cleaning detergent from the tube and relieve the pressure in the system after each use.