Not just looking, but watching?
If I understand you correctly, "I am watching you" implies that you are watching all my actions, in order to know everything I do, and how I do it, right? Or even, you may be spying me (so you might do it secretly)?
If yes, then your choice is
to watch (in this context) = наблюдать за + instrumental
or
следить за + instrumental (this one may also imply spying).
So we get: Я наблюдаю за тобой. or Я слежу за тобой.
смотреть is just "to look"
If you want to say "I am looking at you", it is "Я смотрю на тебя".