This is how to talk about an action that started in the past, and is still happening now:

I've been taking kickboxing classes at my neighborhood gym.

Whenever you use the word "since", you have to use the perfect form "have done":

I've known him since he was born.

We've been ranked #1 every year since 1990.

You shouldn't use the simple past tense ("knew", "were") with "since".