Just follow the steps below
#grub
grub>md5crypt
Password: ******
Encrypted: $1$jxcdN0$hVHViq1aiPf8FziuGJGZp0

/*Copy down encrypted password: */
$1$jxcdN0$hVHViq1aiPf8FziuGJGZp0
/*quit grub now by entering following command*/
grub> quit
#vim /etc/grub.conf
Insert encrypted password in between "splashimage..." and "title...":

Save edited fileand quit

:wq

#reboot

---done------

Now u cant edit the grub menu with to enter and modify u need to giive the grub password by pressing P