How do you edit boot file in Mac?

For the Mac OS X Version 10.5.5 how do you edit the boot file? How would you change the the OS, the computer loads? I just need to get access to the boot file. For example, in Windows XP, it%26#039;s the boot.ini file. Or you can simply change it by typing msconfig in the run box. How do you do it for the Mac?|||I%26#039;m not really good at using macs, double click the file and click open with a different program and look for notepad.