Some people navigate through pages using the tab key rather than the mouse. You must ensure that the order that page elements are presented is logical and gives a similar experience to that which would be obtained visually.
Try to ensure that the default tab order is the most logical route:
To an extent, this will be dictated by your design;
The default order is the order that elements appear in the underlying code that makes the page - specify the order if it is not clear from the design itself.