After you go to the webpage of the game, there are 3 ways in which you can start playing, each of which starts with providing the game with a beatmap (or chart).

Playing through uploading files

You need to upload a beatmap from your file system.

You can also specify an audio file for the music. If there is no audio file specified, the game will use that specified by audioUrl in the head of the beatmap.

In the Discord channel, you can find many beatmap files (it is the temporary location to publish beatmaps until the beatmap store is developed).

Playing through browsing the store

The function is under development.

There will be a store (under development) for beatmappers to publish their beatmaps.

Playing through selecting from history

The function is under development.