You can create a new task either in a contact's record or in the Tasks tab. To learn more, check out this article.

Whichever option you choose, a box will appear allowing you to enter in all necessary information, including a check box to mark tasks as High Priority. 

On the day the task is due, you'll see a red flag next to tasks you've marked as High Priority. This lets you know at a glance which tasks should be addressed first!