Try an even mixture of white vinegar and water in a sandwich bag, grab a rubber band, and head to the bathroom. Put the sandwich bag over the shower head so it is completely submerged and fasten it with the rubber band — leave it there overnight. Take it off, and your shower head will be cleaner and the water pressure should improve.
Do you have other areas of calcium build up?
Use the vinegar mixture on that with a sponge or rag.
Baking soda can clean the sink, and denture cleaning tablets can be an effective toilet bowl cleaner.