What should I do when I get a new follower ?
Getting new followers is excellent!
This means that your art, or the collections you have created, are of interest, that's a good thing! When you upload new artworks or add new images to your collections, the people who follow you are informed and can see what's new in your account
You have nothing specific to do! You may visit the profile of the person who follows you and follow them back (only if you find their work or his interesting collections).