Approve a follow request
If your account is set to private, anyone who wants to see your photos will have to send you a friend request that will appear in your News "You" feed, which you can then approve or deny.
To approve/deny the request, just tap on the username of the user who requested you. You'll be brought to their profile page, at the top of which you should see the "approve" and "deny" options.
Note that if you accidentally denied a follow request, you will need to ask the user who requested you to request to follow you again.