If a channel ability requires a target, you may not activate it without a target just to discard the card.

Discarding the card is part of the cost to activate a channel ability.

The owner of that permanent chooses whether it goes on top or bottom. Other players get to know what they chose. In some unusual cases, including melded or mutated creatures, a permanent may consist of multiple cards. In those cases, the permanent's owner puts all cards on top or all cards on bottom. They choose the cards' relative order, and other players don't get to know that order.