fatal error:undefined function get_header()

i am new to wordpress.i had to convert my static site to wordpress site.everything was working fine i had my index.php done,i created my navigation menu.here is the plot twist i had to link those menus to other pages like contactus menu to contactus.php,when i try to do that i had put get_header() function on top of contactus.php page,when that contactus.php page is loaded it gives the error fatal errror:cal to undefined function get_header(). . .. 🙁