You must be an Office 365 Administrator to activate Microsoft Teams.
Begin my logging into the Office 365 Portal with your credentials and then clicking/tapping on Admin to get started.
Under Settings click/tap on Services & Add-ins in the sub-menu.
Look for Microsoft Teams under the list of add-ins. Click/tap on it to get to the settings for Microsoft Teams.
Toggle the On/Off switch to On and that is all you need to do so that your users can take advantage of the Microsoft Teams Preview.
No other changes are necessary but there are additional options that can be configured.
See the following screenshots for a review of those items which are pretty self-explanatory.
Microsoft Teams - General Settings
Microsoft Teams - Teams & Channel Options
Microsoft Teams - Calls & Meetings Options
Microsoft Teams - Messaging Options
Microsoft Teams - Tabs Options
Microsoft Teams - Bots Options