There are a few apps, that with minimal effort in your day to day, plants trees for free. Check out the apps below and start planting trees.
Ecosia
The first app I would recommend is a search engine that uses the ad revenue to plant trees, so when you click on an ad it funds tree planting . You can install it as a chrome extension on your pc or laptop.
You can also install it from the app store on your phone.
Treekly
This app plants a tree a week for walking 5000 steps a day for 5 days a week. If you can manage that, then a tree is planted on your behalf. This is a great way to keep you active and plant trees. It can connect to fitbits and other step trackers, or it can use the step tracker on your phone. This is available at the app store.
Tree App
Tree app allows you to plant a tree a day for free. All you have to do is watch a short advert and choose where you want your tree planted. This one is the easiest of the apps to plant the most trees. This is also available on the app store