Nested Case How-To

Hi, I'm not new to SQL however I get confused when I transfer from different DBMS like Oracle, SQL Server, Firebird, MySQL and Access.

in SQL Server T-SQL, How do I go about a nested select case like the ff: And I supposed to use this as source for a Cross-Tab.

CREATE PROCEDURE SP_SAMPLE(
@DateStart datetime,
@DateEnd datetime,
@Param1 char(3),
@Param2 char(1),
@Param3 decimal(7,4))
AS
BEGIN

SELECT
Field1,
Field2,
Field3,
CASE
WHEN FieldSomething = 0.0000 THEN '*'
WHEN FieldSomething < @Param3 THEN '_'
ELSE
CASE @Param2
WHEN 'A' THEN SomeOtherField1
WHEN 'B' THEN SomeOtherField2
WHEN 'C' THEN SomeOtherField3
END
END Alias1
FROM SampleTable
WHERE UPPER(Field2) = UPPER(@Param1)
AND Field1 BETWEEN @DateStart AND @DateEnd
ORDER BY Field2, Field3, Field1

END

it has this error:

Error converting data type varchar to numeric.