Need help

This is my first time growing and I am trying to keep them alive but not sure if I got a green thumb. Anyway not sure even what kind of plants I'm growing the seeds came from some loud I had smoked. I got 3 going 2 are tall and lanky 1 is short an squatty. I don't think I've been taking enuff care of them judging by what I've read. They are approx 4 wks old and are not bushy and the leaves are turning yellow. Can some one please help me I don't want them to die or is it too late even?