You will be impressed by beautiful mosques wherever visit in Turkey with ther long minarets and richly ornated small balkonies on which were serving the muezzin (person who sing the call for prayer) heard by whole village in older times.
You can visit the mosques out of praying times covering the hair, shoulders and legs if you have summer clothes. In most of the mosques you will find scarfs.
If you are there for praying you can always enter and join the local people.
The fountains that you are going to see outside is for ablution that is an obligation to wash up face, hands, arms and feet before praying five times a day.