Once you are done with the installation, open your Visual Studio 2017 IDE. When you open it for the first time, it will ask you to sign in. Be sure to sign in with your MSA (Microsoft Account) ID or a work/school account that best represents you. If you don't have an existing account, you can create a personal account for free.

Here is the screenshot of the sign-in screen of Visual Studio, where you need to provide your account credentials:

Not only will signing in help you extend your trial license, but it will also help if you installed the Visual Studio trial version. By default, it will have a 30 day trial period. Once you sign in to Visual Studio 2017, you will get an additional 90 days to try it.

If you have an account that's associated with the MSDN or VSO (Visual Studio Online) subscription, signing in to Visual Studio will automatically unlock your installation. It will also connect to services such as Microsoft Azure and Visual Studio Team Services in the IDE without prompting again for credentials for the same account.

If you missed logging in to Visual Studio at the first start of the IDE, you can do so later from the Visual Studio by using this path: Help | Register Product | Sign in, or by clicking on the Sign in link available at the top-right corner of the screen, as shown in the following screenshot:

Unless you sign out, you will be automatically signed in to Visual Studio whenever you start it. This will synchronize all your changes to the settings in the cloud.