If they know you have a plan/method and intent to kill yourself, the state will most likely take you into protective custody. If they think you lack either one of those, it is less likely you will suffer that intervention.
Posessing a prescription drug (N) without a prescription is illegal, so yes, they would most probably confiscate your N.
Just to add my voice to the chorus, I strongly advise that you delay your decision to CTB if you have any doubts about it. As so many people here have wisely advised, suicide is not a decision that can be undone. Be wise, and explore every other avenue before that final one.
That said, be smart, and don't let your explorations lead you into a worse place. Approach therapists/psychiatrists cautiously, without giving them any reason to incarcerate you or medicate you against your will --such medications can lead to you finding yourself in an even worse situation than you're in now. Forfeiting agency over your life to the medical industry will not make you feel better in any way, shape, or form.
Furthermore, don't cut off your own line of final retreat: hide your N somewhere secure in case that final avenue becomes your only good option. I don't know your situation, but perhaps get a safe deposit box and, if you don't want the temptation, mail a good friend the key for safekeeping. But the upshot is that knowing you can CTB if you need to will often, paradoxically, take the edge off needing to. (A therapist taught me that, actually.) So as much as I would love to see you close that door and stride forward into life without N, you might have an easier time stepping away from the bus stop if you don't get rid of your N.
Strength and courage, @dolphin. Recovery is a difficult road, but if you can find the start of it, it's one worth exploring.