KTuner.com

I needed to get a Ktuner for my 2011 Honda Fit because I recently have been modding the air intake/exhaust.

My OBD2 error has says I'm receiving in too much air, making my engine run lean. The computer compensates by adding more fuel to the mix, but it kills my gas mileage.

How should I go about learning to adjust my air to fuel ratio with this software?

There are several online courses that can teach you tuning theory. There's also information on the strategies you're looking for. Your vehicle was Mass Airflow (MAF) from the factory.