How to change system calls with modules ?

Hello to all,

I'm new in kernel development, and i'm try to learn kernel development. But i have problems in change syscalls, can someone show me how can i change system calls (hijacking) ? My english not good, but i can read all codes. Can someone write example module for change a system call (sys_open, etc..) to writed myself system call ? (my_sys_open etc..). I hope you understand what i mean.

Best regards