There are multiple options of how to navigate to the Activities areas of Microsoft Dynamics 365.
Activities from a View
Select any of the main modules (Sales, Service, or Marketing) from the Navigation Bar, then select Activities.
From here you can pick views of activities to look at. Out-of-the-box, this screen will default to My Activities, which lists all open activities that you own. To change the view, click My Activities to open the View Selector. Within this drop down you can select different views based on Activity Type.
Activities from a Dashboard
You can also see the My Activities view from various out of the box dashboards. For example the Marketing Social Dashboard, Customer Service Representative Social Dashboard, and Sales Activity Dashboard which are the out of box default dashboards that load when selecting either Marketing, Service or Sales from the top Navigation Bar and clicking on Dashboards.
Specific Record Activities
You can also view activities within an individual record. To do that, open a record such as an account that has activities associated with it. When a Contact or Account record is viewed, activities can be viewed by clicking on the activities tab in the Social Pane. Activities can also be filtered to hone in on specific activities.