Put rows of data into a variable in SQL

[Bebe]

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

I keep getting this error when trying to return a select statement as an output variable.

for example:

SELECT @var = (SELECT item_id
FROM #Table
WHERE order_item IS NULL)

I need to have the rows that come back from this inside a variable -- how do i do this; can I?