Is it normal to spend 90% of time in front of pc screen or phone screen.

It's normal, but it's unhealthy. I used to be the same way and the health problems that come with it will take it's toll on you eventually.

Well, that would depend on your job and lifestyle. If what you do for a living involves using a computer for designing/programming/rendering or any other thing that necessitates a pc, sure. If you're on the computer for the sake of wasting time then no. In any case, it helps to go outside you get some natural vitamin D.

Thats my life. All I do is sit on my butt and watch YouTube videos all day, play Call of Duty Black Ops 1-3 Zombies mode, and write stories on quotev.com. Oh, I also listen to music, text people who do nothing with their life, and hang out with my YouTube compainons.

You're missing out on the other joys of life. Staying online is easier and less painful than three-dimensional life challenges, but in turn it is less rewarding.

90% sounds like a exaggeration that doesn't even leave enough time for sleep. Whats so compelling that you need to be on a computer or phone so much?

I find it hard to believe that you squeeze sleeping, eating, showering etc.into just 10% of a day.

Get a Motorcycle.

Normal these days. You're going to want to look into execises to fix forward head posture. Abductor stretches will help save your legs.