If you post what you have, we'll try to help. What you're trying to do is pretty straight forward I think, something like this:
Create proc usp_DoSomething as
--Sample proc to update a table
set nocount on
update a set a.somefield=b.somefield from table1 as A inner join table2 as B on A.primarykey=b.primarykey where b.someotherfield='whocares'