Mark Loftus

I currently work full-time for a UK-based company and I have been offered the potential for some part-time work for a US based Company. This would be on a 'when-required' basis, and would be a second income. I understand the rudimentories of what would be required if I were to become a contractor, but what will I need to do in regards to Tax and NI if I take contract work as a second job, and am being paid in US Dollars?