to be completely honest, if you don't believe in any religion, there's really no purpose in life...i hate to break it to you.
BUT, try getting into some hobbies. Try new things. Things you think you might not like. Things maybe seem difficult to do. Write songs, stories, or make beats. Watch youtubers as they might inspire you to do something. Try cooking. Try googling for fun hobbies. Anything. Even things that people around you might frown upon (Just not the things that might do harm to yourself). Explore the world. Go to far away places (but make sure you can come back to your home). Edit videos or photos. Just do something.
Once you discover that you actually like something, your mind will automatically make that your purpose. I know this because, I didn't have a purpose back then. I cam across youtubers that deal with anything technology. Now, I'm a Tech Enthusiast, and I live for Tech. I will do anything to get tech that I like.