Start by downloading the Obsidian app (it's just on the Google play store), then open it up.
Select "Create" and choose a location to put it (you will probably want to note down where you choose because you'll need the location to be able to sync with your PC) then you'll have a blank new obsidian vault to work with.
In order to set up flashcards there are a few things you'll want to do, start by going to the settings menu (bottom of the left sidebar). Go to "Community Plugins", disable Safe mode, then click "Browse".