database connection was not established yet that's why this 
error is occuring.

$con=mysql_connect("localhost","root","password") or die
(mysql_error());
$link=mysql_select_db('test',$con);
if(!$link)
{
	die(mysql_error());
}
"root" is the user name
"test" is the database name

First you have to make a database connection. coding is below mention:
first u have to create a connection to server like:
mysql_connect("localhost","root","");

then select the database like:
mysql_select_db("database_name");

localhost is default server name.
root is default user name.
"" is default password. it means password is blank.

database name must be wrong or you can use IP address 
instead of 'localhost'.