Q: how can i register a windows tablet to my itunes store in order to load my bought music?

Which one of the Surface devices is this? Does it run the full version of Windows? If so, did you install iTunes on the Surface? If so, then you would need to authorize the copy of iTunes for that Apple ID. You do that by going to iTunes, then Store, then Authorize Computer. See this support document. Authorize your computer in iTunes - Apple Support