If you don't want for someone to make changes to a part while you are working on it then you need to select the part in commonspace and right_click and change status/ locked. And then you are the only person that can modify and check-in the part.
PS: Remember to unlock the part once you are done.
If you've defined authorization on folders in CS, open Preferences and click Checkout, then in Automatic Status Setting area choose Without Checkin Authorization option and set it to Read Only. If you don't have checkin authorization on folder in CS, all the checked-out parts in that folder will have the Read-Only status.