To enable the ToDo list feature in your GMail account, go to Settings->Labs and set the Tasks to Enabled and Save the Changes.
To start using this feature, Gmail will now first reload your Gmail account and come back with this feature enabled, so what can I do with the Tasks feature?
First thing, browse to some mails sitting in your inbox, and check a few of them then select the “More Actions” dropdown and select “Add to Tasks“.
This will open up your Tasks pane on the right and add the mails there. (You can also access the tasks by selecting Tasks on the left hand menu bar.
This is a very simple feature but can be very powerful for those users who keep their GMail open in their browsers the whole day.(the people who live in their browsers) 😉