It very much depends on you. I personally think most people should start with New Who, because it’s much faster paced and The Doctor starts out a lot more likable. I personally started with New Who, then started mixing in Old Who during Capaldi’s first season. Which turned out rather nicely, because early Capaldi actually resembles the First Doctor in many ways, and it was interesting to see how elements of the show unfolded.

For reference, Old Who starts with An Unearthly Child (1963.) New Who starts with Rose (2005.)