I learn by searching the trees around my house and school, I described them to Google and searched images for similar pictures, then I checked the name of the tree in the picture.

Eventually the differences between different kinds of trees became more and more clear, and the names began to make sense in my head...on my way from home to school I would call out the names of all the trees I saw (my friends were very patient with me), and if I crossed paths with an unknown tree I would repeat the search process.

Also I found some local landscape guidelines that identified the local trees. Also my mom would tell me the ones she knew, and on the plant market (I live in Mexico City, near Xochimilco, a part of the city known for being very fertile, where you can find several plant markets) I would ask the people selling the plant a for their names, and eventually I learned them..

TL;DR, googling + asking other people + local tree guides