Transportation is one of the largest sources of CO2 emissions, especially from personal vehicles. When you take public transport it releases slightly more CO2 per vehicle, but spread out over a large number of people. Overall, it releases less than if everyone had driven. Carpooling also reduces the number of vehicles on the road, which can significantly lower emissions.
EVs produce zero tailpipe emissions. Though their production has a higher initial footprint, over time they can reduce lifetime emissions by up to 50% depending on the electricity source used for charging.
The electricity grid is also a major source of CO2. Renewable energy sources like solar and wind emit little to no CO2. You can ask your energy provider if they offer any green energy programs, to power your house with renewable energy. You can also install solar panels on your house or help install solar panels in community project in your area.
While individual efforts help a little, system wide changes only come from environmental policies and are very helpful. You can vote for candidates that support renewable energy and conservation. You can support environmental legislation. And you can join local advocacy groups.