I am new to Zoho and have a problem in populating related fields,

I have "Form A" that includes Student ID,Student Name,Student description. "Form B" includes a sub form "Subform1".

This "sub form 1" refers to a Form C that includes fields,

" Student Id -- Dropdown (Look up field)" 

"Student Name --- Single Line Field "

"Student Description -- Multi Line field" 

Now, What i want is, When is select the student ID from drop down, related information of student has to get filled in Student name and Student Description  dynamically.

Can any one please help me how to implement this populating fields depending on drop down