SQL Server Management Studio is not available
to connect to local server and default instance just use any of these ., local, localhost or YourMachineName. 

If you are trying to connect to a named instance on your local machine just use any of the above followed by \InstancenName.

i.e  .\InstanceName, local\InstanceName, localhost\InstanceName or YourMachineName\InstanceName.

Note

If you want to connect to the default instance you just use the machine name, if you are trying to connect to a Named Instance then you have to use the machine name and the Instance Name.