You can read more about the environmental impact factors in EnergyPlus and the fuel factors as well as all the possible outputs (seems to be associated with site energy) from the I/O reference. There are several ways to report the outputs, I will mention some:

1. Through the standard annual report: In the Output:Table:SummaryReport object, make sure to add the predefined reports "AllSummary" or specifically "EnergyMeters" and look under the energy meters subsection after you run the simulation.
2. Through Output:Variable and Output:Meter objects: You can check the Report Data Dictionary (.rdd) and Meter Data Dictionary (.mdd) files to see all possible variables and meters that you can report based on your model. You can search for the keywords "CO2" or "emissions" and select the desired variables/Meters.

Here is an example of using both output variable and output meters, both should show the same results (of course you can change the reporting frequency):

Output:Variable,
    *,                       !- Key Value
    Environmental Impact NaturalGas CO2 Emissions Mass ,  !- Variable Name
    Monthly;                 !- Reporting Frequency

Output:Variable,
    *,                       !- Key Value
    Environmental Impact Electricity CO2 Emissions Mass ,  !- Variable Name
    Monthly;                 !- Reporting Frequency    


Output:Meter,
    ElectricEmissions:CO2 ,  !- Key Name
    Monthly;                 !- Reporting Frequency

Output:Meter,
    NaturalGasEmissions:CO2 ,!- Key Name
    Monthly;                 !- Reporting Frequency