legal advice on selling online product within USA

Hi, we have an online business and we just need some legal advice on how we can sell our product in the USA and we are a Uk based company. Including, tax situation etc. These are the questions that need answering: 1. We are selling a global service. This service whilst linked to our global product will be just enjoyed locally initally. We are therefore selling a local service to a local market but charging on line by our UK company. Is this allowed and how should the set up be to ensure that this remains legal? 2. Do we need to charge local sales tax? 3. How do we encorporate these points into our terms and conditions?