Functions with dynamic queries

Hi,
I would appreciate it if somebody could send me some examples
(or url) on how to create functions with text parameters, use these
parameters to build a dynamic query and return a value obtained from
that query.
A very simple case would be:

Create table people (nif    char(12),name    varchar(40),primary (nif)
);

And now to create a function to return "name" value giving
"nif" values as a parameter.

Thanks

--
Gabriel D.