The が indicates what is performing the verb. に is often used to show indirect objects or destinations. Take a look at these sentences and notice what each particle follows.
1 – ケンさん が パーティ に きます か？ Will Ken come to the party?
2 – けんさん が きます か？ Will Ken come?
3 – パーティ に きます か？ Will [somebody] come to the party?
が marks the person who will come.
に marks what they are coming to.