This forum is for income tax questions, not sales tax. I can give you some basic information to get you started, but you will have to look elsewhere for details.
Sales tax for online sales is very complicated. Sales tax is charged at the location where the product is delivered, not where the supplier is located. Each state has different rules about whether you are required to collect sales tax, and different rates. In many states, the sales tax rate is not the same for the whole state. There are local sales taxes that vary depending on the specific location. A particular item might be taxable in some states and not in others. So you have to answer three questions for each sale.
- Are you required to collect sales tax for the state that the product is shipped to?
- Is the product taxable in the state that it is shipped to?
- What is the sales tax rate at the location that the product is delivered to?
To collect and remit sales tax in a particular state, you are generally required to register with the state.
Shopify has some features to help you with sales tax, but they don't seem to handle everything. Look for information about sales tax in the Shopify Help Center, and search the discussion forums or post your own questions there.