Locking rows

Hi.I'm using postgresql 8.0.3 under win2000 and developing with VS2003 (npgsql net provider).
I would like to start a transaction with a sql function.
When user press "edit" button on my form, i would lock the current row.
After user has modified data on form, pressing "save" button I would save the modified row by sql update function and so commit.
Well.The update function works fine.
My problem is to lock the current row.
My code is something like:

CREATE OR REPLACE FUNCTION "public"."new_function" () RETURNS SETOF "public"."table" AS
$body$
begin;
select * from table where field = 'value' for update;
$body$
LANGUAGE 'sql' VOLATILE CALLED ON NULL INPUT SECURITY INVOKER;


But I get :

ERROR , BEGIN IS NOT ALLOWED IN SQL FUNCTION

What's wrong?
Please tell me if it's a syntax problem
maybe have I to use pgsql ?
How can I lock row in pgsql ?
Thanks in advance.