This is a great idea if you want to take up acting but did not join a drama club in school.
Improvisation classes are a good place to start. Improvisational acting trains you to pay attention to cues and respond quickly. You'll learn to naturally react to others' prompts and silence your inner critic.
When you advance past basic acting classes, you can take specialized classes for film or stage acting, which require different skills. Experiment with both to figure out which suits you best.