There is more than one option for that.
Much depends on the precise definition of "you" and "someone else" and how you and that "someone else" need to work with the database.
A validation field option can reject all changes to a field once it is not empty.
You can set up one layout to which only you have access where you can edit the field and your "someone else" can be set up to use a different layout where behavior settings deny browse mode access or you can place a calculation field on that second layout that displays the contents of the field that you edited on the first layout.
A script trigger can be tripped on OnObjectEnter to move the user's focus to a different field if the user is not "you". (But drag and drop can still modify the field in this case...)