Terry Im (aka KRNFX) teaches you how to beat box in this tutorial. To begin, he breaks it down into three basic sounds: the bass drum, the high hat and the K snare.
For the bass drum, KRNFX says that you should think of the letter B, without vocalizing it, while pushing the air out of your mouth. This sound will need to be as tight and as crisp as possible.
The second sound, the high hat, think of the letter T and S together and push the air out of your mouth tapping your tongue against your teeth.
Finally, for the K snare, you will need to relax your mouth and suck the air in so it sounds like the letter K.
Don't worry if you can't make these sounds right away. Like everything, it does take practice. Since you know these basic sounds, you can make a beat and in a matter of time, your mouth will be able to imitate a drumkit or a super sonic sound system!