That's a very challenging question and searching those keywords here and on Google will get you a lot of content to review.
As someone who has run a NFL team-based fan site, I'll offer a few things I wish I knew when I started some 13 years ago...
1. Brand yourself. Easier said than done but, based on the look of your site, it's the default style and doesn't make me think New York or sports. But brand goes well beyond look of the site. You need to think about your "voice". Do you want this to be a lightly-moderated site where members can be rabid fans and say/do just about anything or are you wanting to hit a certain insightful fan who can talk sports with good insight? Are you wanting a board which is focused hard on sport topics or be a place fans can talk beer, women, cars, etc. with being a fan the common bond?
There are a lot of good Jets/Giant boards out there already. I'm sure there are a lot for the other individual NY sports as well. So ask yourself "why should anyone come to my board?" I'd be sure to visit all and see what you like and dislike. See if you can offer something they don't or do it better. Define what you want your board to be as if you were creating a business. Once you nail that down, then make sure everything (mod style, look, topics, etc. reflects that).
2. Attract visitors. I'll assume you already participate on boards since you're starting one. I'd continue to do that and ask any friends/mods you have in mind to do the same. If their contributions on other boards draws attention because it's quality content, and they are allowed to add a subtle url to their sig, then some will come to check you out. But realize you only have one quick visit to impress and you need to have something going on that makes people want to join and tell a friend.
3. Be patient. I don't know of any sports-related board which popped up and went big fast without investing a ton of money or resources into it. Growth is generally a slow process. You can try to run contests, do giveaways, interview athletes, etc. to generate short-term interest but, bottom line, the content has to be interesting and updated regularly to keep people returning and, eventually, wanting to be a regular part of a board.
Good luck.