best if you read it yourself but the bullet points are
offsetting is a last resort not a checkbox for feeling good and going on with your life, you should first try to decrease your footprint and use this for things you cannot (yet) change
Some offset companies are more trustworthy/effective then others
When buying an offset, experts say to check to see if it’s verified by a third-party agency such as Climate Action Reserve, Verified Carbon Standard, Climate, Community, and Biodiversity Standard, Verra or the American Carbon Registry; the RainForest Alliance also verifies some offsets, as do other environmental groups.
Cool Effect is a verified and well-respected site for obtaining offsets; it includes an easy calculator so you can see how many tons of carbon you need to offset per flight. Offsets purchased through verified Terrapass can be made using its tool to calculate your overall carbon footprint for you car, home energy and air travel. Carbonfund.org, also verified by third parties, takes another approach, letting customers choose packages of offsets based on overall lifestyle, vehicle model, type of home or even based on the size of an event.