How i can theme checkout page?

I need to theme the checkout page. I have set content, account, billing, and payment panes respectively, inside the same checkout page. Now i want to theme the page creating tabs for each pane and also need some jQuery work inside.

Actually i want ajax based form filling and introduce multi step form inside that page.

Is there any --.tpl.php file to create? Anyone please suggest me where to start, what file to create and where to place?

Thanks n regards

Posted: Nov 23, 2011