The Skateboard was a weapon and vehicle that was dropped during the development of Grand Theft Auto: San Andreas. Though the player can still use it as a weapon through modifications, the weapon is nowhere in San Andreas. The skateboard was meant to be introduced to the player in the mission Madd Dogg's Rhymes. The skateboard is developed enough that it is specified to which surface is skateable. However, in the gta3.img file (unless opened from IMG Tool), the animation file for the skateboard exists.
To use the skateboard pick it up and select it from the weapons inventory items. Press ~k~~PED_ANSWER_PHONE~ whilst over it to pick it up if your inventory is full.
To use the skateboard, select the it from your weapons inventory. Use the movement controls to steer the skateboard. Tapping ~k~~PED_SPRINT~ helps you pick up speed and ~k~~PED_JUMPING~ makes you jump.