Planting more trees isn't the right solution. We should cut CO2 emissions, thing that is not so easy to implement when you have superpowers (like USA) who oppose such changes...
Certainly both courses of action would be beneficial to the environment. There is agreement among 99% or so of the scientific community that this needs to be done, or we face very serious consequences.
At the same time, trees absorb CO2 and convert it into Oxygen. Deforestation continues to outpace reforestation and tree planting, so the easiest thing you can do as an individual is to plant more trees. And it doesn't have to cost you anything...find some trees, look for their seeds on the ground and bury them a couple inches under the soil in various places.