get the value from text box

Question

hi,,,this my query as given below

select * from tblEmp
where convert(datetime,convert(varchar(10),a.DueDate + <days>,101)) <= convert(datetime,convert(varchar(10),getdate(),101))



i want to get the values of <days> from textbox 
to execute this query and bind data in a grid..how can i do this.
please provide me code by example..
thank you