Thankfully, services are not taxable in most states. However, taxes can be complicated, and you should take the time to understand the rules as they apply to you and your particular situation. We've found this link from TaxJar can be helpful.
Unless your service is creating a product you are usually safe. Products are typically taxable in a state if you are considered to have a "nexus" of business in the state.
The transfer of knowledge or skills is not taxable - yet! :)