You have to start charging Sales Tax when you meet "nexus" limits in each state. They are different in each state, some will be $100,000 of sales in a 12 month period and others with be 200 transactions in a year and in most it will be a combination of the two. You should use a tax program etc to start tracking your sales as they will help warn you when you are near nexus in a state and then you can register with that state when you hit nexus and start submitting those taxes. A good resource I've found is a youtube channel called "ledger gurus", they are selling a service, but they also go over everything you need to know to feel more confident in understanding the process. Of course you really should talk to an Accountant to get the best advice for your specific situation and confirm the product/tax codes you need to use.
A lot of this changed in 2018 after a Supreme Court ruling regarding Wayfair, that allowed states to start going after remote sellers for Sales Taxes who did not live in their state.