When filling out your contact form you may find that the tab button doesn't go to the next box and instead skips around and goes out of order. This is due to the arrangement of elements in the (left) site panel of the editor. To set your form up to tab from field to field, order the elements for the input boxes in order from first to last in the left panel.
(for larger image: Right Click>Open Image in New Tab)
The first input box should be above all other input boxes for the contact form, with the next input box underneath that one.
For more information on setting up the contact form: