George Monbiot compared the impact of eating a leg of lamb with a trans-Atlantic flight. His initial estimate suggest the environmental impact of the two was similar. Actually the lamb of leg has slightly less impact, but the point stands. Lots of the small, everyday things we do can have a big impact, especially when taken cumulatively.
Personally, I think the most important things that all of us should be doing are
A) stop constantly consuming, especially meaningless plastic crap, cheap gadgets from Amazon etc
B) Stop driving everywhere