The first line of a command sign must always be [command].

The second, third and fourth lines of the sign will be the command that you want to run.

Example sign, which runs the /help command for the user:

[command]

help

To edit a sign, right click the sign you want to edit, while sneaking and holding a sign item in your hand. This will open the regular sign editing window which will allow you to make the necessary changes.