Some magazines buy all rights. That means you can never sell the story again unless you get the rights back. Some magazines pay on publication. That means you won't be paid until your story appears in print. This could take a year or more.
Many colleges offer classes as well.
Group Bench W is an online critique group. The organization is legitimate. Opinions on the value of the lessons varies. The Society of Children's Books Writers and Illustrators is a national organization for writers and artists. Anyone can join.