I just installed SQL Server 2008, and I wanted to use the command editor to execute queries.
To do that I opened the command prompt, and I typed
Sql
The server you are specifying cannot be found or is not accessible. so either the server is wrong (have you spelt it correctly) or you are not specifying it correctly or it is not accessible from your machine.
After you've got that right, you will need to specify a username/password or a -E command qualifier for a Trusted Connection. You may also need to specify the database with a -d qualifier.
You can get help by typing
Sqlcmd -?