WITH  q AS
        (
        SELECT TOP 1 *
        FROM    mytable
        /* You may want to add ORDER BY here */
        )
DELETE
FROM    q

Note that

DELETE TOP (1)
FROM   mytable

will also work, but, as stated in the documentation:

The rows referenced in the TOP expression used with INSERT, UPDATE, or DELETE are not arranged in any order.

Therefore, it's better to use WITH and an ORDER BY clause, which will let you specify more exactly which row you consider to be the first.