First Download the "Visual Studio Team Services" extension from the marketplace, it allows you to connect to Team Services, monitor your builds and manage your pull requests and work items for your Git repositories.
After you've installed the extension, open either the root folder or a sub-folder of the cloned Git repository. Once a Team Services or Team Foundation Server 2015 Update 2 (or later) repository is detected by the extension, you will need to provide your credentials (if credentials weren't already found). If you are required to provide your credentials, there will be an indicator in the status bar whose message will indicate that you need to login in.