create log file in magento?

Put any message on log file ..
Go to Admin > System > Configuration > Developer > Log Settings
Change the value to YES for Enabled field.

Mage::log("Write you're message here. those you want to write on log file");
This message will be display on log file(root folder/var/log/system.log)

if you want to create you're own log file ...
Mage::log("Write you're message here. those you want to write on log file or any variable", null, 'filename.log', true);
Here 4th param means force logging
You're custom file will be create on var/log/filename.log


NOTE:- For Log file 2 things required..
1) Developer mode must be set to true
2) Logging must be activated in the system configuration.

you can find log method on app/Mage.php file in line Number 784.

public static function log($message, $level = null, $file = '', $forceLog = false){}