Multiply taxable value with applicable sales tax rate. For example, if you are selling any product or service for $100, then you can calculate sales tax by multiplying 100$ with sales tax rate. In New York, average sales tax rate is 8.1892. Multiply sell value with 8.1892 to get tax amount.
Further, you should check tax rates applicable in each county to arrive at correct calculation. Each county has separate sales tax rate along with state and city tax rates.