If I join a family plan, can the other person see my photos or other stuff which i save in the icloud?

No, not even if you share an iCloud storage plan. See the following from Share an iCloud storage plan with your family - Apple Support

When you share a storage plan, your photos and documents stay private and everyone keeps using their own accounts with iCloud—just like when you have your own storage plan. You simply share the space available in the iCloud storage plan with your family members, so you only have one plan to manage.

The exception to that would be things that you intentionally share (also true if you are not Family Sharing).