How to put header in Tiga theme?

Hey there,
you can do this pretty easily. Look at your theme’s header.php file for clues. That theme may also come with a way to change the header file. Refer to any available documentation the theme author might provide.

Additionally, have a look at this article: http://codex.wordpress.org/Function_Reference/get_header – While a little more advanced, it shows you methods of controlling that header area of your theme.