Assuming you are working with SQL Express
By default SQL Express is installed as a named instance of SQLExpress
you can connect to it using localhost\SQLExpress or <ComputerName>\SQLExpress
as server name in management studio express (computrename is hostname of your computer)
you can find your local instance (default or named) by opening SQL Server configuration manager (Start-->Run-->SQLServerManager10.msc) or from command prompt
running
SQLCMD -L