could u be a little more specific about your problem...
Or do u have to start yet and asking for design tips?
Use a dataTable to store data of shopping  cart in, so u can project it to a datagrid...
u can u a redirection between catalog page  and cart  page...
In catalog page u
{
 add  data to dataTable
redirect to cart page
bind dataTable to dataGrid
}

i am using datalist to display products from my ms access table.
when i click addtocart button.. i want to perform add  to cart  as a
function in the same page.. so can u tell how to implement  the
function.

You could use eiter Session or Cookie. Or you could temporarily stor into
database related to Session ID. I think, most of the shopping  carts are made
with cookies.

Look at Petshop application available from MS ... it uses a hash table which
is then stored in Session.