"Actively seek correct information on climate change and your role in causing it, and take action accordingly.
This answer may seem annoyingly vague. The problem is that no one action is best for everyone.
Most of the global warming since 1950 is very likely to have been caused by the sudden spike in greenhouse gases: mostly carbon dioxide from burning fossil fuels, but also methane from cattle and other sources. By now carbon dioxide concentrations have shot up from 290 to 390 parts per million. If we continue 'business as usual' we may easily reach 1000 parts per million or more by the end of the 21st century. This will give the Earth's climate a push whose rapidity and magnitude has no known precedent."
So, what's best to do depends a lot on who you are. If you're a politician, maybe you can help eliminate some of the $300 billion worldwide yearly subsidies to the fossil fuel industry subsidies that waste taxpayer money and encourage the burning of carbon. If you're an average American, taking one fewer round-trip flight from New York to Los Angeles can save 3/4 of a ton of carbon dioxide. If you're a subsistence farmer in Africa, you can turn some agricultural waste into charcoal and bury it: biochar is a promising low-tech way to sequester carbon. In many places, but not all, you can try to kick out politicians who fail to take climate change seriously. The best ways to do this vary from place to place.
Thus, broad generalizations are difficult. But for everyone, the key step is to start actively seeking correct information on climate change and your role in causing it, and then take action accordingly.
Some places to start include the Union of Concerned Scientists' webpage:
the National Academy of Sciences's free online book:
and, especially good for businesspeople, the McKinsey & Company report on paths to a low-carbon economy:
It is also wise to compute your carbon footprint and think of ways to reduce it:
For a version of this answer with links to the facts and numbers, and a lot of other answers, see: