Grab the Reins:
Choose one - Until end of turn, you gain control of target creature and it gains haste; or sacrifice a creature, then Grab the Reins deals damage equal to that creature's power to target creature or player.
Entwine (Choose both if you pay the entwine cost.)
Question: if I choose to pay the entwine cost, do I have to announce who or what I'm going to target with the second part of Grab the Reins?
All targets are announced when the spell is played. Please note, the targets are what to take control of, and what to deal damage to. There is no targetting involved in what to sacrifice, that is simply a choice made on resolution.
Siege-Gang Commander:
When Siege-Gang Commander comes into play, put three 1/1 red Goblin creature tokens into play.
, Sacrifice a Goblin: Siege-Gang Commander deals 2 damage to target creature or player.
Question: does the comes into play ability go on the stack? As in, can i kill Siege-Gang Commander with a Lightning Bolt when the ability is on the stack so he cant sacrifice his tokens and shoot me?
It is a triggered ability that makes the token so it does use the stack. Triggered abilities can be recognized (usually) because they start with "When," "Whenever," or "At."
Also, in the future, completely seperate questions on completely seperate topics should be completely seperate posts.