From OpenTTD

Signs are a method of placing messages on the game map.

[edit] Placing Signs

Placing a sign is done through the Landscaping Toolbar. Click on the button and click on a square that you want to place the sign on. A dialog window 'Edit sign text' will appear, allowing you to type something more-or-less useful in. Press OK to store the sign.

[edit] Editing Signs

To edit an existing sign just click on it to bring up the 'Edit sign text' dialog window again.

[edit] Deleting Signs

To delete an exisiting sign all you have to do is edit it, and clear all text.