Your pressure washer will have an inbuilt hose with the lance attached for cleaning, but you will need a regular garden hose to connect it to mains water. This hose will need to be long enough to allow you to get the pressure washer in reach of everything you want to clean.
Some people worry that having a longer hose will decrease the pressure to the machine, but this is not the case. As long as water is flowing to the machine, all should be well as it is pressurised at the washer itself.
If you have issues with the pressure, check there is not an air lock in your machine. Some machines won't start if this is the case and you will have to press the trigger when the washer is not turned on to release any air. If you still have issues after doing this, check all hose connections at the mains and the unit itself and also check the hose for any leaks or holes.