A. There are several products on the market that are very effective at removing limescale, such as Viakal or Limelight, but ensure you buy one with a spray function to allow easy application to large vertical surfaces, and always follow user instructions and wear rubber gloves.
Once applied to surface of shower door, leave to work for a few minutes, then remove with a sponge. Heavily soiled areas may need gently scrubbing with the coarse side of a scouring sponge, taking care not to scratch the glass. Check the usage instructions on any product before use, and be particularly careful with limescale removers if you have chrome fixtures on your shower.