Creatures you control gain trample until end of turn. (A creature with trample can deal excess combat damage to the player or planeswalker it's attacking.)
Draw a card.
Crash Through affects only creatures you control at the time it resolves. Creatures you begin to control later in the turn won't gain trample.
You may cast Crash Through even if you control no creatures. If you control no creatures as the spell resolves, you'll just draw a card.