I remember very little, but these teenagers, possibly early 20's were on a ship and in one episode, they touched some leafy plant that turned them into the aliens that they battled. I was only about 5 (between 1994-1996) when I saw it on TV.

I should also mention that they were on a spaceship and that the intro was like a monologue with some kindof panorama around the ship

It could be TRIPODS. I think it was first shown on TV in the mid 80s. It could have been repeated in the 90s but not sure. You can always check the imdb to see if it jogs any memories.