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';