I'd start local(which I have done, but not been ultimately successful due to wildfires running over my planted areas) - research how to germinate seeds of local trees in your area, collect the seeds, plant, and give the results up to God - some will grow, some won't; some will be eaten, burned, or mowed, but you'll be learning and develop more advanced strategies over time to see what works.
Plant in National Forests, alongside trails, greenways, and/or highwaysides. Most trees/shrubs are much better than grasses for preventing erosion, so planting on cuts and hillsides of highways, etc. would be twofold productive.
I do it this way because I am too cash-poor(southern California does that quite easily) to give much time or money to charities right now.