How could a teen get dragged into a murder?

It's super important for a writer to come up with their own ideas, especially a crucial plot point like this. If you're stuck however, I'll try my best to inspire an answer in you. :)

Think about yourself. What would provoke you into murdering someone (or hurting someone)? What if they hurt somebody you loved? What if they surprised you and it was a physical reaction (impulse) that happened without thinking? Or maybe the teen is drugged (by somebody or even just drunk) and doesn't really know what she's doing (can't think reasonably)?

Now think about this for a second. How guilty of a murder are you if you don't do something that could stop it? For example, you know that in thirty seconds a large piano is going to drop out of the third floor of the building, yet you pass someone you dislike going that direction, and don't warn them. You didn't necessarily DO anything wrong, but are you guilty for their death by piano?

Or think about recent news stories. I saw one recently about a teen girl that convinced her boyfriend (who you know has got to have something already a little loose in the brain) to kill an innocent girl that she felt was competition for herself. The boyfriend did the killing, but is she more guilty because she schemed and pressured him into the killing?

Anyways, I hope this has your brain juices flowing and that your story works out! Good luck!

You want a character in your book to be guilty of murder and convicted of it? Uh, why not have her murder someone then? The most obvious scenario would be for her to murder her boyfriend, or someone else close to her. There's a very fine line between suicide and homicide, so if someone's not in the right state of mind it's not hard for them to "take down" the people who are around them. In fact, most murders are committed by a family member or close friend, and spousal murders are the most common of them all. Perhaps her boyfriend cheated on her -- or at least she thinks he did -- and she murders him before trying to kill herself?

However, you should take into account the age of your teen. If she's under eighteen, she won't be sent to prison, she'd be sent to a juvenile detention centre. This doesn't mean she'll get a lighter sentence -- if anything, it'll have no impact on the inevitable life sentence -- but juvenile detention centres do differ from prisons on some aspects, so it would pay to do your research.

Are the readers on the girls side or the opposing?

If we're meant to like this girl then you could always blame it on drugs or maybe an 'Imaginary friend' made her do it if you're more on the paranormal side of things.

If we want her to go DOWNN, then she can just be some crazy person in dire need of some meds.

Where will this scene occur? Is it the inciting incident, or the first plot point, for example?

You should think of it, sounds like a crucial plot point in your book, no?