A fellow NY'er? Welcome! Others will certainly know better than me but it is my understanding that a 3.5/170 makes you very competitive for Northwestern! Feel free to reach out on the board with any questions.
Honestly, I would just jump right into the course. If you would like a book to compliment the teachings, check out the LSAT Trainer by Mike Kim. A lot of 7 sagers recommend it. Good luck with your studies and welcome to the community!