Whenever you are logged in, the card menus will include a top-level "follow" link. Click that link to follow that card.
follow all cards of a given type
Navigate to a Cardtype card, and then follow the same procedure as above.
unfollow a card
If you are following a card, the menus link will read "following" instead of "follow". When you mouse over it, it changes to "unfollow". Click that link to unfollow the card or cards.
see what cards a user is following
There is not yet quick interface for this, but you can go to |user's card|+*following to see a Pointer card that indicates which cards a given user is following. When a signed-in user clicks "follow" or "unfollow" anywhere on the site, this card is automatically updated. It can also be edited directly like any Pointer.