PL/PGSQL - How to pass in variables?

Using PL/PGSQL, I am trying to create a procedure to display the
count of rows in any single table of a database. The End-user would
pass in a table name and the prodecure would display the table name
with the row count.
I am able to hardcode the variable for table and get the appropriate
results from my count function (see below), but cannot pass in a
variable and have the function work. Any suggesstions???

CREATE FUNCTION get_table_count(tablename text) RETURNS integer AS $$
DECLARE

--tablename ALIAS FOR $1;

rowcount INTEGER;
BEGIN

SELECT INTO rowcount count(*) FROM tablename;

RETURN rowcount;

END;
$$ LANGUAGE 'plpgsql';