You did not specify what you were doing, and die from a blood clot due to inactivity.

I am trying to think of what to do.

While you are thinking of what to do, a rip in the space time continuum occurs, causing a war between alien cyborgs and football zombies on planet Earth. As you realize what you want to do, a alien cyborg fries your body as a football impales your brain.
Don't ask how that works, but you are dead.

I sat in my isolation box, filled with fresh, never spoiling food and water.

You realize that the only food that doesn't spoil is honey. You are allergic to honey, but you shrug and eat a little anyway. 
You have a severe allergic reaction, to the point where you must go to the hospital. But there is no hospital in your isolation box.
You crawl out of the isolation box, begging those nearby to take you to the hospital. And after a while, a helpful soul comes, picks you up, and drives you to the hospital.
You are at the hospital. Your symptoms are starting to go away, and you believe it's due time to leave. So you do just that.
You take a walk to soothe your body after your allergy attack. But there is pollen nearby, and you're allergic to that to. You breathe, not realizing it's there. It only gives you a minor rash usually, but for reasons unknown, you suffer another allergy attack. This time, you don't survive.
You are dead.

I am drinking water.

PokemonBoy217 : Okay, but that water contained Carbone Dioxide, which is really bad for your health. This kills you. Haha, you're dead! I'm using my laptop.

You inject it, and you didnt know that you misstaken it for poison that kills you in 10 seconds after taking it, in first 5 seconds you start to feel lots of pain. Than you realise that you took poison and hurry to inject vaccine for death, but 10 seconds pass,you die and drop the vaccine. It smaches and vaccine that took your whole life to develope is gone...   And yea, you are dead!

I am shinigami, and I wonder around in human world.

djura23 : you care about a human extend his/her timeline ultimately killing yours(i know death note enough to know how to kill a shinigami)
 
 i am cheating in a poker game

One of the people sees you cheating, and stands up. He clenches his fist, and punches you square in the face. You are down, but not out yet. The man who knocked you out picks up your unconscious body. He ties cinder blocks to your arms and legs. He takes you to the nearby river. He gets you on the bridge and pushes you down. You wake up just before you hit the water. You try to get yourself upright as you enter the water. And you do. The pressure from the fall breaks the cinder blocks taped to your legs.  However, you break your legs with them, and the cinder blocks on your arms remain in  one piece. The man who tried to drown you comes down there with a gun and begins shooting. You try to swim away, but the cinder blocks on your arm and your broken legs prevent you from doing that. He successfully shoots you five times. You are in pain. The pain is mild at first, then begins to swell. The pain becomes overbearing, and you die from it. You have died.

I am doing my homework.

PokemonBoy217 : You are doing your homework. You work so hard you pass out on your book. As you lay there sleeping you drool off the side of your book. The drool runs down the side of your book and then proceeds to pool on your desk. There is so much of it it begins to drip off the edge of the desk. The drool drips onto the gnawed piece of electrical cord of a space heater in your room. It creates a short and the heater bursts into flames and the carpet catches on fire. The fire spreads to the curtains of your window creating more smoke and the smoke alarm in your room goes off waking you up. You see that your room is on fire and you try the door but, someone has blocked it off as a practical joke to make you late for school in the morning as usual. Your room gets hotter, more firey, and smokier. Then you remember the other window which is small on your other wall. You break it with a paper weight and smash the rest of the glass out of it then climb out...getting outside safely. Then you are attacked by the rabified dog who heard you smash the window and came to see what was up. He bites you in a main artery then leaves and you fall to the ground and bleed to death. You are dead.

It is 6:07 am. I write the next users name in my death note.

Because you don't know how i look, it doesnt work.I track down your IP adress, and find out where you live. I come to your when everybody was a sleep, use my lock pick to get in, take your death note. I write in your name and cause of death is stabing yourself with a small knife. You cant kill yourself because knife is too small, and beg me to finish you off, I refuse and wait until you bleed to death so i can cut up your body and put it in a bag. I sink the bag in the deepest lake i find... YOU ARE DEAD!

I know Im about to die soon, so i just drink, eat and play games whole day.

djura23 : you die from a horrible death of....cheese dun dun duuuunnnnnnn thats right you got poisoned by bad cheese and you threw up your organs

i am playing with a blunt wooden sword

A wizard comes out of a bush and turns your blunt wooden sword to a sharp diamond sword with wood colored spray paint on it. You have a sword fight with your friend, and accidentally stab him in the chest, killing him. Knowing what you've done (as well as what the wizard has done), you decapitate the wizard that transformed your painless wooden sword into a bloody diamond murderer. You then proceed to  commit suicide, realizing that is how it starts, before the wizard's magic  wears off. About halfway to the heart, your diamond sword becomes a wooden sword. Due to this, You can't kill yourself with it anymore, and you beg those around you to finish the job. But nobody does so, causing you to bleed to death. You are dead.

I'm doomed, so I buy $20,000 worth of pasta to celebrate.

You choke on the pasta, and it is rotten. It poisons you and you run our of breath while choking. You are dead.

I am replying to a thread...

At that specific time you reply djura23 was tracking a random IP address and it so happened to be yours and
he tracked you down and came to your house and killed you. Instead this time, with a... rusty....... SPOON!!

I'm setting in the dark, cause I can.

The darkness consumes you. You are dead.

I am in a bus, alone aside for the driver, and its raining in the bus.

zanderlex : You feel something on your face. Then your mouth. Suddenly, you realize the bus crashed and has a hole in the cieling, and you are starting to choke on the rain. The rain consumes your insides and tears through your body before I can save you. I'm so sorry. You are dead.

I'm slowly progressing through the day...

Yes but just as your day comes to an end you start to realize your phone rang you pick it up
and a voice says you will die in 7 days and then you're all like "Don't prank this number again
or you'll die in 7 seconds" then you hang up and 7 seconds later you notice that
the stranger on the phone was a recorded message of you and you realize all this time you were never
alive the phone call was from you when you were alive. The Inception begins.
The end.

I'm watching Netflix cause that's all I look forward too.

This one is bland..

You became hypnotized by the movies you were watching all day. You eventually saw a horror movie that had too much blood, gore and violence. That movie gave you a heart attack and you died on your couch, you are dead.

My pet dolphin seems to be really hungry..

Your pet dolphin seems to be having some kind of reaction (,AN ALLERGIC REACTION) to the food you gave him. The dolphins stomach is so disturbed, the dolphin makes excrement all over your floors. The excrement are really disgusting and they smell terrible. You try to get out of your house, but that doesn't work; they dolphin already covered the door in excrement as well.

You are trapped.

You have been in the room for hours and your nose has already melted off. The droppings are so poisonous, they enter your body and they consume you lungs. You choke and you collapse to the floor. You are dead.

I need to go to the toilet really bad.

After 7 years, 11 months, 3 weeks, 6 days, 23 Hours, 59 minutes, 59 seconds, and 59.9 miliseconds, you burst and launch into Space, but have no Helmet, or Space Suit, so you die of Suffocation.

I fly a Chopper over the Appalachian Mountains.