java script function for selecting value into the textbox?

Question

hi,
i am using datagrid to display user information and i take checkbox column into the datagrid and i take one textbox and textmode is multiline. i am taking contactname, mobileno...... etc columns to the datagrid.
      my aim is when i select checkbox that row mobileNO value is display in Textbox and again i select another checkbox that row mobileNO is also display in textbox with comma seperation, and i deselect checkbox that row mobileNO is deleted from textbox.
                   how to write javascript function todo this. please give a sample code. i am not know how to write javascript functions
please give a solution
Thanks.