You can play Glowsynth in three ways:
First, you can press the keys on your Qwerty keyboard. For example A, W, S, E play the notes C, C#, D, D# and so on. To toggle the current octave to "high" or "low", press the "-" or "+" key. (The page loads in "high" mode.)
Secondly, you can click and drag the notes with your mouse (or tap with your finger).
Thirdly, if you are using Chrome, you can connect a MIDI keyboard, and play via that method. If it doesn't work first time, try refreshing or re-connecting.
If using the Qwerty Keyboard method, here are some example tunes to get you started. Use the comments form below to post examples of your own!