Water droplet marks are a result of mineral salts in the water etching the glass. Small affected areas can be polished out with jewellers rouge but it's very hard work.
There are companies that provide a professional service to fix the problem but it is expensive.
Glass actually "cures" with age and is at it's most vulnerable when it's new. It follows then that the first few months of use is the time to avoid letting water droplets stand on the glass for any length of time. A squeegee after each shower works well.
Washing weekly with a strong detergent (not abrasive) followed by windex and a soft cloth will keep your shower glass like new.