its difficult to give a definitive answer without knowing your current database structure, but here is my approach.
Hope it is clear
When you have one category per item, you can store the categoryid as a field in the items table, and everything is straightforward.
With this structure, you can only have one category per item, (unless you duplicate the item).
To have multiple categories per item, you need a separate table with a one-to-many relationship with the items table, with itemid as the foreign key.
(You could add a primary key to this table, or create a compound key of the ItemID and CategoryID fields)
so your table could look like..
ItemID : CategoryID
1 : 1
1 : 2
2 : 1
Linked to the items table, this would mean item 1 belongs to both categories one and two. Item 2 belongs only to category one
When inserting a record, your form would need to show all categories available, using either a multi select list or check boxes, and have a multiple record insert behavior acting on this category table.