When you open the WebLily editor (www.weblily.net/web/weblily/runlilypond), you can see lilypond source code in the left window. The right window shows the music representation of the source code as compiled by lilypond.
To get started, locate the pitches bes'2\p a'4\< c''4 b'2\f in the source code on the left, and try adding a space and new pitch (say g'). The music window on the right will be updated automatically shortly after you have entered the pitch. If the music window shows an error, you can either click the link in the error and jump to the offending source code position, or you can use the undo button to go back to the last correct piece of source code.
Another thing to try is loading a template, for example "Piano and Melody with Lyrics" from the drop-down box. After loading, WebLily displays the initial template in the music window. Next, locate the variables "text", "upper" and "lower" in the source window and try adding new notes and text. Again, the music window will reflect the changes you make after a short delay or inform you about an error.
If you have tasted blood, you can learn more about the lilypond format on lilypond.org.