When do i need to charge sales tax?

You need to charge sales tax when you have nexus (taxable presence) in the customer's state either physical nexus (office, warehouse, employees) or economic nexus (exceeding the state's revenue or transaction threshold, typically $100K or 200 transactions). Once nexus is established, you must register, collect, and remit for that state. Most US states have economic nexus thresholds following South Dakota v. Wayfair.