How to create a social media app for Android to store images and docs

I am very interested in creating a small social media app for the android platform to allow users to share and store images and docs. Compared to a lot of social media apps, the one I am trying to implement will allow users to edit the images of others before reuploading.

Anyways, the reason why I have come to this forum is because I don't know where to start.

I know I need to use the Android Studio IDE and I have Java experience. The only thing I am unsure about is the server/backend. What would be a nice open source storage? What else would I need to consider?

It would mean a lot to me, even the world if someone could explain to me how I would go about implementing this idea, like what tools and languages to use like Java, SQL etc.

Thanks