How can I stop thinking and shut my brain off?

Find an activity you enjoy. For me it's baking. It requires my complete concentration and it helps if you're listening to your favorite music at the same time. It really takes my mind off of everything.

Sleeping its a matter of how I go to sleep this is why its hard for people to sleep because their brain is constantly working

Meditation, listening to soft music and focusing on the lyrics, yoga, breathing exercises, running/exercise

It's not thinking that's the problem, It's negative pessimistic thinking. What you should try and do is think positive or distract yourself; in a healthy way. If you can train yourself to think more positive then you will eventually have no problems with your thought process.

Try to picture something incredibly plain, like a piece of paper with a black dot on it. imagine the paper and dot as clearly as you can and think only of the boring paper and dot. imagine getting closer and then further away from the paper and the dot, and try to imagine this all incredibly vividly only focusing on these 2 simple things.

try meditation :) or physical exercise.

You should never wish to shut your brain off. but to reduce thinking you can sleep or talk to people about your experiences

Try focusing on something like your breathing this way you will be thinking of nothing but your breathing

Music. Try to find a song that 1)understands you or 2) keeps you distracted

If you feel overwhelmed by certain thoughts, self-distraction is the natural way to deal with it. Children are usually expert at distracting themselves, and as adults we still have this ability even though we may not be aware of it, and even though we may not use it when needed. Distracting yourself typically changes your thoughts, and as your thoughts change your emotions also change. How to distract yourself? It can be as easy as walking over to another place, or going outside, or doing something practical, or taking with someone. Basically anything that takes your mind off the current issue that is occupying it. Distracting yourself gives yourself a little breathing room and may make it easier to deal with the issues that are so challenging.