need Unix script help

This is Unix script assgnment in class. I have a students list named:classdb.txt

ID:NAME:Course1:course2
001:Harry:Comp123:Comp234

I have to write a script to add new student in the list.
I think should use the "sed" command, but is not working.
please help.
this is what I have try:
echo enter the students ID
read num
echo enter the students name
read name
echo enter the course name
read class1
echo enter the course name
read class2
$sed -f classdb