Drop down or combo box

I have spreadsheet with employiee names & classes. How do I make a dropdown or combo box work to bring them in from a central list.

Example:
Employee Name: Employee Number Class Code

(use one drop down to fill in the above list.)

Note: List of personel are on seprate spread sheet, broken down by same colum headers listed.
Ie -Employee Name--Employee Number--Class Code

Please help thanks