DECLARE @SearchType int
DECLARE @SearchText varchar(50)

SET @SearchType = 1
SET @SearchText = 'A'


SELECT
*
FROM
[TableNameGoesHere]
WHERE
(@SearchType=1 AND [Field1] LIKE @SearchText + '%')
OR
(@SearchType=2 AND [Field2] LIKE @SearchText + '%')
ORDER BY
CASE @SearchType
WHEN 1 THEN [Field1]
WHEN 2 THEN [Field2]
END