How to add tabs to a form?
Use the Field Group Module. It comes with default HTML wrappers like vertical tabs, horizontal tabs, accordions, fieldsets or div wrappers.
Field Group:

Fieldgroup will, as the name implies, group fields together. All fieldable entities will have the possibility to add groups to wrap their fields together. Fieldgroup comes with default HTML wrappers like vertical tabs, horizontal tabs, accordions, fieldsets or div wrappers.
The field group project is a follow-up on the field group module in CCK. The release will only exist for Drupal 7 release and higher, so since the existence of the Fields API in core.
By moving field group to a separate module, this may open some new perspectives.