If you have been using TimeMachine to keep your computer backed up that's the best tool for migrating to the new computer. When you first turn on your new computer connect the TimeMachine hard drive to it and near the end of the sign in process you'll be asked if you want to transfer data. Yes you do.
If you aren't using TimeMachine go out and get yourself an external drive - it should be at least 3-4 times as large as the data it will be backing up. (If you are using 100GB of your hard drive you'd get a 500GB external drive.) You need a backup - it isn't a question of if your drive will die but when. Plus, of course, having the backup will make moving into your new computer so simple.