"include ("include_pages/db.php") "
correct, BUT:
include works relative to the current directory. so if your script was in the 'myphp' directory, you'd have to do
"include ("../myphp/include_pages/db.php") "
with the '..' to go up one directory, andthen into the include_pages dir.
There is no difference between using include on a shared server and on a 'non-shared' server. All you have to do is give the correct filename, either by giving the entire path from the filesystem's root, or a relative path from the current directory.