After WSUS installation you need to configure it. Open Server Manager, click tools, scroll down and click Windows Server Update Services
If you haven’t specified the updates locations during the installation, then you will see the following popup. Choose the updates location and click Run. After a while, when it finishes, click close and the WSUS console will open. Click two times Next.
Select the upstream server for the updates. The first option, it synchronizes the WSUS server from Microsoft. Select the second option if you want this WSUS to connect to another WSUS server. Click Next.
Select to use a Proxy Server if the server needs it to connect to the internet. Click Next.
Next, click Start Connecting. It takes a while. It downloads update information. Click Next.
Choose in what languages should be downloaded. Click Next.
Select the products to be updated. I selected all. Click Next.
Then select the update classifications you want to download. Click Next.
Choose when to synchronize the update and click Next.
Select Begin initial synchronization, click Next and then Finish.
And now WSUS is synchronizing.