Best way to improve poor quality images?

Not being a PhotoShop guru or a designer of anything artistic, I was wondering if there was a "simple" way to best increase the quality of "old" camera phone or other digital images so as to clear up the noise and pixelation in them.

Side note - with the prevalence of digital photography reaching a decade or more of popularity now, maybe there is a business model in cleaning up old digital camera photos taken in the pre-iPhone days.