when i selected an item in combobox then relatd GUID is saved in table?i want code plz

hi,
Iam working in windows application in C#. i have a problem,i have 2 data table.in that tables autogenerated columns is there. the tables are
1)ITEMMASTER
2)ITEMCATEGORY
GUID,CATEGORYNAME,CATEGORYCODE these are column names in ITEMCATEGORY tables
GUID,ITEMNAME,ITEMCODE,CATEGORYCODE,PRICE these are columns in ITEMMASTER table.
data is saved in ITEMCATEGORY table.no problem.
here i have a problem
in ITEMMASTER form i have
Textbox1=ITEM NAME
Textbox2=Item code
ComboBox=CATEGORYCode // Combobox filled with ITEMCATEGORY table
Textbox3=Price
when iam click ok button,data saved in that table,but in CATEGORYCode column
which item i selected that related GUID is saved in that column.
THIS IS MY PROBLEM PLZ HELP ME ,I WANT C# CODEING
Thanks in advance
himanshu