First Need to decide where you want to create a public folder and you have sufficient access on the folder inside which you wanted to create a Shared Calendar

1. Log into your mailbox using Outlook,Expand until you get required folder
2. Right-click on it and select New Folder in the pop-up menu.
3. Type in a name for the Shared Calendar, select Calendar Items in the Folder contains drop-down list and hit OK.

Right-click the Shared Calendar folder, select Properties.
Go to Permissions tab.
Hit Add and choose users and/or groups you wish to grant access permissions to.
Select required permissions level in the appropriate drop-down list and hit OK.