In the calendar list on the left side of the page, hover your mouse over a calendar and click on the 3 dots icon to the right of the calendar name, then select Settings and Sharing.
Scroll down a bit, to the "share with specific people" section. Click "Add People." Enter the email address of the person you want to share your calendar with and select the desired permissions.
From the drop-down menu on the bottom, select a level of permission, then click Send.
Click here for a Google guide to getting started with Calendar
Click Here for a guide on switching from Outlook to Google Calendar. (Helpful for Outlook power-users)