Example to populate Combo box

Can someone point me to an example?

I have WPF MVVM program.

on the main screen I have 3 text boxes a combo box and button. When user clicks on the button a popup screen shows up and displays a data grid, when user double clicks on any row the pop up hides and program populates the text boxes with the data from the data grid row.  I am not sure what is the best way to populate the combo box with the data from the database.  I can get the data from the database and populate the combo box in the code behind module, but I don't think that's the correct way for MVVM program, I am coming from Windows Forms world and I am pretty new to WPF and MVVM.

Thank You.

Peter