I have an sql query "Select * from tablename" whose output is

col1   col2       
  A     1 
  B     2 
  C     3

I want to modify the above output to below as following

A      B      C
1      2      3 

Please let me know how could I achieve this

You will need to perform a PIVOT. There are two ways to do this with PIVOT, either a Static Pivot where you code the columns to transform or a Dynamic Pivot which determines the columns at execution.

Static Pivot:

SELECT *
FROM
(
    SELECT col1, col2
    FROM yourTable
) x
PIVOT
(
   min(col2)
   for col1 in ([A], [B], [C])
)p

See SQL Fiddle with Demo

Dynamic Pivot:

DECLARE @cols AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @cols = STUFF((SELECT distinct ',' + QUOTENAME(col1) 
                    from t1
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = 'SELECT ' + @cols + ' from 
             (
                select col1, col2
                from t1
            ) x
            pivot 
            (
                min(col2)
                for col1 in (' + @cols + ')
            ) p '

execute(@query)

See SQL Fiddle with Demo

If you do not want to use the PIVOT function, then you can perform a similar type of query with CASE statements:

select 
  SUM(CASE WHEN col1 = 'A' THEN col2 END) as A,
  SUM(CASE WHEN col1 = 'B' THEN col2 END) as B,
  SUM(CASE WHEN col1 = 'C' THEN col2 END) as C
FROM t1

See SQL Fiddle with Demo