Sql queries giving low performace need to improve it

Question

Hi,
 
I have 3 table a,b,c.
 
select * from a
 
select * from b where aid in (select id from a)
 
select * from c where bid in (select Id from b where aid in (select id from a))
 
these quries in sp and giving correct result but as performance I need to optimise these
could you please suggest how do i improve perfomance. is any tools available to optimised
sql queries.
 
Thanks