How to write Store procedure

Hi,

I am new to using store procedures Can anyone please check and tell me whats wrong in it

create procedure Poll_Grid
@top int,
@ofset int,
@orderby varchar(50)
as

if (@orderby = "question")
 select top @ofset substring(question,1,50) question, startdate, username,categoryname from polls
 join users on polls.userid= users.userid
 join category on polls.categoryid= category.categoryid
 where pollid not in (select top @top pollid from polls order by Substring(@orderby,1,50))
 order by substring(@orderby,1,50)
else
 select top @ofset substring(question,1,50) question, startdate, username,categoryname from polls
 join users on polls.userid= users.userid
 join category on polls.categoryid= category.categoryid
 where pollid not in (select top @top pollid from polls order by @orderby)
 order by @orderby

I want to check if @orderby is passed as question it should fire the first query other wise last and also I want to dynamically insert the Ofset and top value for my paging require ment

Please help..