the easiest is to use Setup Assistant. when you first turn the new mac on it will offer to migrate you applications and users from another mac or a backup drive. ollow the instructions there.
the backup needs to be either a bootable clone or a time machine backup. you did not say what kind of backup you've got. it will not work of you simply copied some files to an external by hand. f
you can also do it later using
Migration Assistant. It's located in /Applications/Utilities.
however, keep in mind that while using those automated tools works very well for most people you may have some issues because you are migrating from PPC to intel. consult this link for details
http://discussions.apple.com/thread.jspa?threadID=435350&tstart=0
this link also suggests that you should migrate manually but that is not actually necessary in most cases.