Why on earth would you need a spell to put someone in a coma? You're so young surely you can't have made an enemy you deem worthy of such a fate.
Furthermore while spells may appear easy to do (simple materials, simple chant etc) I can assure you they are not, if you haven't mastered the basics your spell is pretty much guaranteed not to work. You don't get to wake up one day, light a candle say some words and think you're a witch who can do anything you want, it doesn't work like that, it takes years and years of continued study
Lets assume for this purpose that you have the above, then there is also the consideration of personal responsibility for your actions, you get back what you put out.
Either way this is a really bad idea and would suggest a different approach, like talking to the person in question or avoiding them if that doesn't work