Help on Trugger/Function

I am new to postgresql -- I am trying to create a trigger on a table --
I get this error on the function
   ERROR:  Unrecognized language specified in a CREATE FUNCTION: 'plpgsql'.
Recognized languages are sql, C, internal, and created procedural
languages.

Here is my function that I get this error on:

create function head_insert() returns OPAQUE as '
   BEGIN
   NEW.tests := 0;
   NEW.fails  := 0;
   NEW.reps  := 0;
  END;
'
LANGUAGE 'plpgsql';