Begin finding a comfortable position, where your breathing is not restricted and you are not hunched over. Lying down is not great for your back, so try a simple 1/4 or 1/2 lotus (full lotus if you're flexible).
The first step to "stop thinking" is the counting method: begin counting each breath (in 1, out. in 2, out), up to 10, and then return to one. This will help you check yourself to make sure you don't have any background thoughts (if you keep going past 10). Focus entirely on the counting. It will take hours of practice to even get to the point where you can quiet your mind to this.
When I first "stopped thinking" I was at a retreat at my Buddhist temple called "Quietness of the mind, the body, and the mouth." We meditated for 10 hours a day, ate for 2, in complete silence, for 3 days. Before this, I had been meditating 1-2 hours a day for a few months. The point is that it takes quite a lot of time to truly quiet your mind.
When just starting out, you should meditate using the counting method for 15-30 minutes a day. Don't beat yourself up if you can't just jump into it. It's a very difficult thing to do.
Best of luck!