There is an open beta available through the Steam interface, which gives you access to a few non-essential features. This version is not required. This guide uses the current beta in examples, so there may be small differences between these examples and your version if you are not using the beta.
To set up your audio for use in the beatmap editor, download and install the free audio editing tool Audacity.
In the top menu bar, select File > Open, and open the audio file for your song.
Make sure there is at least two seconds before the first note that you plan to map in this song. The editor does not let you place notes within the first two seconds of the audio. In most cases, you just need to add two seconds of silence at the start.
In the top menu bar, select File > Export > Export as OGG.
At the bottom of the window, set the quality to your desired number. We recommend 7, since it results in good audio quality without being too large of a file.
Save this OGG file as a new copy, and remember where it is for later.
Never overwrite your original source audio, even if it was an OGG. You will need it if you have to make more edits after Exporting.
This will allow your beatmap to stay in sync with the music. The easiest way to find BPM (beats per minute, aka tempo) and Start Offset is with the software tool ArrowVortex.
The above link is a more convenient .zip version of the tool that is otherwise found on the ArrowVortex website.
If there are several different BPMs that are off by small amounts, this song likely has a drifting tempo. We highly recommend using a song with a steady tempo for your first map, but if you're feeling brave, check out the audio warping guide.
If the song has large intentional BPM changes, it will be much more challenging to sync until the editor supports BPM changes. Syncing a map for these kinds of songs is involved, although it can be done without resorting to high precision note placement in the editor. Check out the Advanced mapping guides to learn how to do this.
You're ready to start mapping your song now. You can explore the editor at your own pace, or if you need a hand, check out the Beginner's guide to Using the Editor.