There are quite a few questions that crop up on this sub that seem like kids hoping someone will finish their homework for them. At least you're being honest about it. I won't do your work for you, but I can point you in the right direction.

The Dutch involvement in the thirty years war warrants extremely long answer. I think the best course of action I can take is to suggest a book that might help you: The Thirty Years War by C.V Wedgwood, rather than paraphrase a topic that can and has filled entire books.

Has your teacher gone over the Eighty Years war before going into the thirty years war? It sounds like she hasn't. In brief, it took the Dutch eighty years to consecrate their independence from Spain, and they were still fighting for recognition at the outbreak of the Thirty Years war. This, in addition to economic competition, both on the european continent, and overseas, gave the Dutch every incentive to challenge Austro-Spanish (a.k.a. Hapsburg) hegemony in Europe; there was no single person that led the United Provinces against the Spanish.

If I had to point out a Dutch leader, I would point to two people; Frederick Henry, prince of Orange, Stadtholder of Holland, Zeeland, Utrecht, Guelders, and Overijssel (and consequentially, de facto ruler of the United Provinces), even if he did die a year before the peace of Westphalia was signed, and Frederic V, Elector Palatine, who although was not Dutch, lived in The Hague during his exile, and continuously lobbied the Dutch government to keep fighting against the Spanish.