Sort a varchar type column (Alphanumeric Values)
i am assuming before - you would have integer value always

declare @temp table
(val nvarchar(99))
insert into @temp values ('1-2'  )
insert into @temp values ('3-5'  )
insert into @temp values ('1-2A')
insert into @temp values ('6-12')
insert into @temp values ('6-7')
insert into @temp values ('6-12D')
insert into @temp values ('20-30')
insert into @temp values ('15-18')
--insert into @temp values ('3')
--insert into @temp values ('53')


QUERY

select * from @temp
order by CAST(LEFT(val,CHARINDEX('-',val)-1)  AS INT)


Exactly Ordered

select * from @temp
order by CAST(LEFT(val,CHARINDEX('-',val)-1)  AS INT)
,CAST(LEFT(RIGHT(val,LEN(val)-CHARINDEX('-',val)), patindex('%[^0-9]%', RIGHT(val,LEN(val)-CHARINDEX('-',val))+'.') - 1)  AS INT),
RIGHT(val,LEN(val)-CHARINDEX('-',val))


after run second query the output would be 



UPDATE AFTER COMMENT: sometime varchar val won't come (3-5) or (1-12A) instead of whole number 3 or 5... at the time the above answer getting an error... how to correct way to sort.

-- DECLARE TABLE
DECLARE @temp TABLE
(val nvarchar(99))

--INSERT INTO TABLE
INSERT INTO @temp VALUES 
('1-2'), ('1'), ('3-5'), ('6'), ('1-2A'), ('6-12'), ('7-8'), ('9-12D'), ('9-12'), ('17'), ('9-12B'), ('19'), ('20-30'), ('15-18')

-- QUERY
SELECT * FROM @temp
ORDER BY
CASE 
    WHEN ISNUMERIC(val) = 0 THEN CAST(LEFT(val,CHARINDEX('-',val)-1)  AS INT)
    WHEN ISNUMERIC(val) = 0 THEN CAST(LEFT(RIGHT(val,LEN(val)-CHARINDEX('-',val)), patindex('%[^0-9]%', RIGHT(val,LEN(val)-CHARINDEX('-',val))+'.') - 1)  AS INT)
    ELSE val 
    END ,val


RESULT: