hi scenario is i have 2 reckoner which i use to block a 6/6 trample creature, can i cast 1st strike and kill the 6/6 creature
another question is can my opent deal damge only to one reckonr and deal 0 damge to the other and deal 3 trample damge to me?
First question: you can absolutely first strike it to death.
Second question: No, your opponent cannot do that. They'd have to do 3 damage to each.
Question one: What cakrause86 said.
Question two: Yes and No. Yes, he can assign all damage to one of the blockers. No, he can't deal exceeding damage to you unless all blockers have been assigned lethal damage.
702.19b The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that's being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that's actually dealt. The attacking creature's controller need not assign lethal damage to all those blocking creatures but in that case can't assign any damage to the player or planeswalker it's attacking.
You don't "cast" first strike. Boros Reckoner gains first strike as part of an activated ability. You activate that ability.