Activities can be created per Container, or independent of a Container. They are more granular than Milestones, as they must be assigned a specific time, and duration.
To create an Activity, you can navigate to the "Activities" tab, then click the calendar icon on the top right to add a New Activity. If you're creating the Activity from within a Container, you can look for the same icon in the "Activities" tab.
Then, you'll get to this screen, where you'll name the Activity, give it a Start and End date, as well as choose who to assign this Activity to. You also have the option to select Responsible Parties, or other Users by name, as long as this Activity is associated with a Contract.
You'll also have the option to make this Activity recurring, set a reminder, and if you'd like to send out an email notification.
Then click save, and you've created your first Activity!