Pistons are powered by redstone 2 blocks vertically above the piston

Place a piston or sticky piston facing any direction.
Place a regular block on top of the piston.
Place a redstone block on top of the regular block.
Break the regular block and the piston will extend.

Also applies to redstone power from levers and redstone torches.

Power changes apply immediately for redstone torches, but levers and redstone blocks require a block update event to trigger before they power the piston.