Pretty much I'm driving a hummer. I get 12mpg or less. Don't small gas anywhere. Check engine light comes on periodically. Don't know what the problem is.

If you perform all of the steps in the link below, your Civic should get 35+ MPG:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563-4.html#post293582

. Check for DTCs--your oxygen sensor and/or Engine Coolant Temperature (ECY) Sensors may be bad and directly affect gas mileage. Jump the Service Check Connector (SCC) with a paper clip, turn ignition to ON (Position II), and count Check Engine Light flashes. The SCC is located in a rubber mount under the glove box, approximately 1" from the kick panel. Use a small screwdriver to carefully pry it out.
. Check engine timing:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-46563.html#post219963

. Replace spark plugs (NGK only).
. Replace air and fuel filter.

My 1993 Civic gets 42 MPG around town and up to 56 MPG on the road.