After water vapor, the most prominent greenhouse gas in our atmosphere is Carbon Dioxide. For Carbon Dioxide, 74% of what we emit is directly related to fossil fuels. Where we burn the most currently is electricity and transportation followed by industry so I would start with those.
-Cut back on electricity (high efficiency light bulbs, A/C adjustments, EnergyStar appliances, etc)
-Switch to electricity options powered by solar, geothermal, nuclear, or wind energy
-Invest in Green Infrastructure (Mosaic Solar, etc)
-Use public transportation and walk when viable
-Get a car with a good mpg rating or switch to a hybrid/electric and use a non-fossil fuel based source
-Buy less manufactured products and recycle what you use (switch to reusable water bottles, make compost piles)
-Make a small garden
-Buy food locally and in season
-Reduce consumption of meat, especially lamb or beef
-Support legislation for stricter carbon emission standards
-Support taxes on gasoline (makes other options more cost competitive)
Overall, it is a hard feat to do a lot as an individual but it all adds up and with awareness and advocacy your impact can be increased. Currently, we are trying to get back to 350 ppm and want to stay below 450 ppm to prevent what is predicted to be a threshold for serious ramifications with rising water levels (would put US east coast cities under water) and more severe weather to name a few. However, this is largely uncharted territory so we don't really know if that is a good cut off for sure. We are currently raising CO2 at a rate of 2 ppm a year. This year was the first year where our annual emissions did not increase. To note though, if annual emissions stay the same we will hit 450 ppm in 25 years or less.