To purchase membership of Clubs, Societies and Media or to pay Hall Subs (where these are administered on behalf of the Hall by the Union,) all you need to do is go to the home page and, if you're logged in, you'll find a "buy button" at the bottom of the page.  If you need addtional products such as AU afflitiation for Clubs, you'll be promopted for these when you go to the cart on the righthand side.