how to transfer data from macbook to mac mini on start up.

The most efficient method would be to make a Time Machine backup of your MacBook and plug in the TM drive to the Mini before you boot it up for the first time. Then when you get into Migration Assistant select the option:

and follow the subsequent instructions.

Please click and carefully read Setup Assistant and Migration Assistant. You have provided no details so I'm assuming that you are migrating from an older MBA to a new MM.