Could you describe what you mean by "under that letter"? What is in
the field you're searching in the table? What do you want to find? If
you could give an example of the data it might be easier to reply.
In your query, for the criteria of the field which you
want to filter on, add this:
Like [Enter letter] & "*"
When you run the query, it'll prompt you to enter a letter
(you can enter more than one letter). Enter a letter and
the query will return records where the field begins with
that letter.