I am assuming that textfield9 is a formfield bookmark name.n  Try
something like:

Sub ScratchMacro()
If MsgBox("Do you like beer?", vbYesNo, "Question") = vbYes Then
 ActiveDocument.FormFields("textField9").Result = "Have one on me."
Else
 ActiveDocument.FormFields("textField9").Result = "Ok, I'll have one
myself."
End If
End Sub

Try writing some code and if it doesn't work, post it here and someone will
help you with it.
http://word.mvps.org/FAQs/MacrosVBA/VBABasicsIn15Mins.htm

In vba help, review the Input Box. (A Message Box doesn't collect
information from the user, just provides it to the user.)

Unless you want the user to be able to change it in the document, you
probably want to set a bookmark for your location in the document, and when
running your procedure unprotect the document, insert your information in
the bookmark, reprotect the document without resetting fields and then
continue. http://word.mvps.org/FAQs/MacrosVBA/InsertingTextAtBookmark.htm 
In talking about protecting and unprotecting the document, I am assuming
that this is a document protected for forms - because of your field name.
This is what Word calls an "online form." Check this in help. For more about
online forms, follow the links at
http://addbalance.com/word/wordwebresources.htm#Forms or
http://word.mvps.org/FAQs/Customization/FillinTheBlanks.htm especially Dian
Chapman's series of articles. You may also want to look at
http://www.word.mvps.org/FAQs/TblsFldsFms/LinesInForms.htm.

Finally, rather than an input box (which calls for a text answer) you may
want a UserForm on which you can put checkboxes or drop-down menus. It is
well worth your time to learn to use UserForms because they are very
powerful and flexible.
http://word.mvps.org/FAQs/Userforms/CreateAUserForm.htm Ms. Chapman's later
articles discuss these as well.