To record a timestamp and a note when changing a TODO state, call the
command org-todo with a prefix argument.
org-todo) ¶Prompt for a note and record a the time of the TODO state change. The note is inserted as a list item below the headline, but can also be placed into a drawer, see Tracking TODO state changes.
If you want to be more systematic, Org mode can automatically record a timestamp and optionally a note when you mark a TODO item as DONE, or even each time you change the state of a TODO item. This system is highly configurable, settings can be on a per-keyword basis and can be localized to a file or even a subtree. For information on how to clock working time for a task, see Clocking Work Time.