Buy the Friday New York Times (available late Thursday night) and check the listings in the back of the Weekend/Arts section.Before you leave, check the websites for Carnegie Hall, Lincoln Center and Town Hall. Lincoln Center's website has a popup menu in the upper right corner that allows you to see all events in the next 2 weeks (or other interval).
Here's a website with links to all concert venues in NYC, including churches, museums, etc:
If you're trying to get tix for a major venue immediately before a concert, don't hesitate to see what the scalpers hanging out front are selling, then see what the box office has before you make up your mind. Often the scalpers have much better seats, and they'll sell them for less than face value if it looks like they may get stuck with unsold tix. Of course, you'll need cash to buy from a scalper.
