Attack a creature you control with instants?
"Attacking" and "casting an instant" are completely different things.

In Magic lingo, "attack" has the very specific meaning of initiating combat. After your first main phase, you may declare creatures to attack your opponent and/or his planeswalkers, and your opponent may then block with his creatures. You are not allowed to attack yourself or your allies, and you may not attack specific creatures.

"Casting a spell" such as Shock means you select a valid target for it if necessary and pay for its cost. For example, Shock requires any one target creature (or player) as its target. Your creatures do qualify because they simply are creatures. There are spells that may target only creatures controlled by an opponent, but in that case, it would explicitely say so on the card itself.