You'll need at least 25 posts and 2 weeks of membership to reach the minimum rank to quality for discount at HH. If you continue contributing, then there are additional benefits as you go along. You can find out more on the link above.
You also have the option of becoming a 'Club Supporter', although this is entirely up to you. For only £25.00, you gain all the benefits straight away.
When calling, go through to the Parts department, and state you're from the club and what your username is.