The pressure washers at Home Depot (even the one I bought) won't work. You need a trailer rig with a hot water unit (mandatory or you won't get jobs), water tank and probably 8-12 gpm minimum washer. Having it all enclosed would be better and more secure. You'll need 10K for even used equipment (not including pickup) plus you'll need a way to get accounts.
You might check around town for places that rent trailer washer rigs even if they don't have hot water and start with the easy jobs such as small strip center sidewalk cleaning. If you're going to go at it for a living and have the required capital, I wouldn't bother with residential work. You also need to research what, if anything, needs to be done with the runoff. If you go to a municipality you aren't familiar with, wash the grease off a gas station's deck, you might find out you are responsible for the runoff and you'll suddenly be $5K poorer.
On the other hand, if you just want to see if the work is for you or your teen, get a 4gpm (minimum) stand alone unit and do the driveways and fences in residential areas. Less than 1K in startup costs and door to door flyers for marketing. I'd budget 1-2 weeks in just flyer delivery. If you decide to go pro, you can always use this unit with small jobs. Once, the office complex I was at had a wooden fence that got tagged. The city came out and removed the graffiti, but the fence was now 2 shades. I brought up my washer and did the rest of the fence to make it look better. Had I been doing that professionally, I would have then wanted to seal-finish the fence. Anyway, those types of jobs you can do with the small units.