I'm visiting NYC for the first time next month and would like to know what attractions I should definately book in advance, and which I can wait on till I'm there. I tried to book a "bluescruise" today and it was already sold out! What about Les Mis, Statue of Liberty, 2-hour cruises, etc? Also, where do I find the best pizza and NYC Cheesecake?

You shouldn't have to book any of the items you mentioned in advance. As for the Statue of Liberty, I'm not sure if the island is open for tourists. If it is, then get down to Battery Park early to avoid long lines. An alternative suggestion for seeing the Statue is to ride the Staten Island Ferry (it's free!). It offers a magnificent view of the Statue and the New York skyline.
As for tickets for Les Miz, don't get them in advance. You can easily get them at TKTS, the half-price ticket booth.

Best Cheesecake is at Junior's in Grand Central Terminal. Sold by the slice and the cake.

Best Pizza, try John's Pizzera, multiple locations, the original is in Greenwich Village. The theater district outlet is the most attractive located in a former church on West 44th Street near 8th Avenue.

Walk across the Brooklyn Bridge and eat the best pizza in NYC at Grimaldi's. Check www.zagat.com or Citysearch for exact location, basically under the Bridge.

You can find the best cheesecake at Junior's. They are located at Grand Central Station, but if you prefer to see the original location, you can just take a train ride into Brooklyn. The rest of their food is pretty good too. (take the N, R, or Q trains to DeKalb Avenue; it's right on the corner once you come out of the train station).

You can try Ray's Pizza down at the Village for Pizza. You can just ask anyone where it is once you're in that part of town.

You shouldn't really need to book anything in advance, unless of course you're going to catch one of the more popular Broadway shows. Then you shold definitely buy in advance. There is no need to buy tickets for the Statue of Liberty in advance, but do check with the Port Authority if the Statue is going to be open during the month of July because of all the terrorists threats.

Have fun in the city.

Best pizza = Grimaldi's on Old Fulton Street in Brooklyn. John's Pizza (W44th or 45th?) and Totonno's (2nd Avenue in the 80's) are very good too. All of these are the thin-crust coal oven/brick oven type pizzas, not the classic NY Style buy-by-the-slice pizza.

Best cheesecake = Junior's in Brooklyn. The rest of the food there is sinful-icious. They've got absolutely AMAZING sandwiches, guaranteed to clog your arteries.