One method for locking records is to add a number field, Lock, to your table. Format it with a check boxes format and specify a custom value list of just one value, the number 1. Then resize the field on your layout so that only the check box is visible. You can put the layout text "Lock" next to it to label it's function.
Then, for each field on your layout except the Lock field, you can find it in Manage | Database | Fields, double click it to open Field Options and specify this validation calculation:
If the check box is empty, you will be able to modify the data in each field. If the check box is selected, you'll get an error message with an option to revert the data. You can choose whether or not to allow users to override this warning.
There are other more sophisticated ways to "lock" a record using either script triggers or Manage | Security and passwords, but this is the simplest option IMO.