A simplistic answer would be below. You declare the variables and then use them. Your declarations should be of the correct type to avoid using CAST/CONVERT in the SQL script to coerce them to the data type you want.
Declare @VARIABLE1 varchar(1), @VARIABLE2 varchar(10), @VARIABLE3 varchar(20)
Set @VARIABLE1='/'
Set @VARIABLE2='33.50'
Set @VARIABLE3='Center'
select t.DOCNBR,t.ITEMNMBR,t.TRXQTY, i.IVIVINDX
g.actnumbr1 + @VARIABLE1 + g.actnumbr2+ @VARIABLE1 + g.actnumbr3, (g.sales * @VARIABLE2)
from InvtMast t
join InvtMast i on i.ITEMNMBR=t.ITEMNMBR
join InvtLINES g on i.IVIVINDX=g.actindx
where i.ITEMNMBR='BASK'
and t.DOCNBR=@VARIABLE3
