Can someone give a link to any good tutorials about how to use DataStores?(for beginners)

Also I would be very happy if there are not only leaderstats, because I am still not sure how to save avatar or inventory

ProfileService is a really good module for data stores. This video is an easy starting point to learn to gain money when touching a brick: video

You can find the devforum post and its API in its description.

Just look for previous questions in ScriptingHelpers, or you can go to developer.roblox.com, or if you want to look into inventory systems go here: https://www.youtube.com/watch?v=ZbX7K-GabA8&t=1160s