Since you didn't mention what kind of tour you'd like to do with a local guide, here are some suggestions:
https://www.alternativeathens.com/
https://www.athenswalkingtours.gr/
https://www.thisisathens.org/withalocal/locals
Note that only licensed guides can accompany and guide you in museums or archaeological sites. You'll find a list with contact information below: (sorted with those for Athens)
https://www.tourist-guides.gr/guides/list-of-licensed-tourist-guides/?bla=&q=Attica
The most convenient area for visiting Athens is Plaka and its surroundings. But by moving a little further away from this area, you'll have other excellent options with fewer tourist crowds while still being within walking distance of the main sites.
If you're used to exploring cities on your own, Athens is ideally suited to this approach. While a guide is helpful for archaeological sites, you can avoid walking tours or food tours and explore on your own with a map and a little research beforehand.
Here's a map of central Athens and its main attractions:
https://www.google.com/maps/d/edit?mid=1OhGNGU1LUEgwFTul8u4YEe_tijF8Hqs&usp=sharing
