Convert existing page to WordPress

We are developing a new site http://poncesuiteshotel.com using WordPress but would like to ‘import’/convert some of the code used in our existing site http://www.poncesuites.net/reservation/selectroom.php. The developer placed all the associated code for this including style.css, session.php etc. in a separate folder ‘reservations’. Can someone please point me in the right direction. I have already created a child theme for the new site. Tried copying the code for selectroom.php into the new ‘check-availability’ page and (of course) got a mix of code and incorrectly formatted content when displaying the page.
I have very limited knowledge of php, html etc but can follow it as I am a programmer (vb.net).