It depends. If you are selling off the shelf (not custom) software and you are both in NY, you definitely have to collect sales tax. It is determined by the county of the purchaser. You then also have to submit quarterly sales tax returns (if you fail to do this, you have to pay a $50 penalty each time).
Services, however, are not necessarily taxed. So it depends on what you are actually leasing or selling.
Regardless, you most definitely need to talk to an accountant in NY state preferably.
Are you registered as a collector of sales tax in NY? If you just collect sales tax without having registered with the NY State (and city, maybe) tax authorities, it could be considered criminal fraud. (Sounds harsh, but that's how it is.)