Ugh, all these answers saying "Don't do it bro" or "It'll get better". At least explain why! Don't worry, I'll actually answer your question.
First, learn a shit ton of things. You have nothing to live for right? Do crazy shit! Parkour on top of buildings! I believe it was Jeb Corliss that actually said in a interview or something. He wanted to do something cool, but also was suicidal. So he chose base jumping. He could do something fun, but at the same time, if his parachute failed, he would get what he wanted.
Also, religion can be a fun thing to play with. I'm not saying "Believe in god, he'll save you", cause honestly, fuck that. Seriously, how can a god who is thought to be "good" be as merciless to kill people, and then priests or the religious tell you when a loved one dies, "He's in god's hands now". Like that's a good thing? It's like loving a serial murderer, who kills you, and now you have to spent eternity with him.
I got a bit off track at the end there, sorry.
Keeping it simple, no Rat Poison. Bad idea. Keep reading...
I answered this question in another post. But here's the answer:
How do you go then?
I think the best methods are either drugs or combinations with drugs.
The easiest method is to go out and buy a bottle sleeping pills. When you go to a pharmacy, look for bottles that either say "sleep" or "PM". I think the ones I have are tylenol PM. They can knock you out in one pill. Plus, two chemicals that can kill you in a single pill. You can buy a 100 count bottle. Take them one by one, or multiple at once.
What you can also do is combine it with other suicide methods.
You can take the sleeping pills, then go into a bag. What you could do is take the sleeping pills, and have the bag be closed with a rubber band, but hold onto the rubber band, so it doesn't close the bag, and when you fall asleep, and move your finger, you will let the rubber band close the bag, filling the bag up with CO2, while you sleep.
Or, you can also go the hanging + sleeping pills route. You can go out and buy a nice rope, but make sure it can withstand your weight. If anyone asks, just say its for a little science project, or construction project. Tie a noose, and tie the rope somewhere sturdy. Place a object that you can stand on a bit to the side of the rope, so when you put the noose around your neck while standing on the object, you kinda lean. Take the pills, and just wait. Eventually, you will sleep (While standing), and fall off the chair or object, and just hang.
This is quite a list of things to do, and I won't really try to tell you to not do it. It's up to you.
Yep. That's it. Rat poison is a shitty way to go, seriously. Poisons in general aren't a good method. Even cyanide, a popular method, is very fast acting, something in the realm of just a few minutes, works by basically keeping your brain from getting oxygen, or also just suffocating you. Nasty.
In some countries, suicide by toxic poisons such as pesticides, are used for suicide protests. It's used cause its a really terrible way to go, and you are in a lot of pain.