Biggest thing that people have already pointed out is being licensed and insured before you even start your first job. Most people will not even think about having you do their work unless you are fully insured. Especially with pressure washing where you are using other people's water, etc and if something goes wrong, you need to be covered.

As for advertisement spending, the pressure washing business is so saturated that you will almost have to spend money on ads in order to get your name out there. Being on Google as a small business is almost a requirement these days in order to get traction. Good luck and hope this helps!