Create great designs! The best way to get more followers is by making art that people love. Make sure to self-promote and upload as many designs as you can!
Also, follow people whose art you want to follow. Following more people will help you connect with more TeePublic users, and will get more eyes on your store.
Looking for some inspiration on how to take your art to the next level? Make sure to check out the Artists to Watch collection: https://www.teepublic.com/stores/artists-to-watch