Popup Window with parameters

Hi

I am new to Javascript and wonder if all you gurus can help me out
there! Help will greatly be appreciated!

I have to pages : pgHelptest with a Question.gif picture on it
and :pgHelpPopup with multiline Text box

Ok what I want to do is create a help control.For ex.You have a textbox
and the user doesn't know exactly what to type in.Next to the textbox
is a questionMark.gif.When you click on it you will open small
pgHelpPopup with a string you passed in from pgHelptest and display
this string in the multiline textbox.

Can anyone please help me with some pointers.

Thanks in advance