Of all the tips that I could give as the first, this should be the most important. As in anything that requires any amount of skill, whether that is music, sports, writing, or fill in the blank: ______, the one piece of advice I will share is this: practice. When you are starting off, you will not be good to begin with.When you are beginning to learn vocal percussion, you will need to start by learning two or three sounds. You will need to practice making those sounds, even if you are not making them in a rhythmic pattern. Once you are able to make those sounds consistently, then you will need to learn to put them into a rhythmic pattern. This, again, is something that you will need to practice. Over time this pattern, along with others that you practice, will become easier to do. It will require less thought and be more instinctual. You will learn exactly where to take breaths, where to insert new sounds and fills, and be able to keep constant tempos and be able to speed them up and slow them down at will.
Vocal percussion, beatboxing, whatever you want to call it takes practice. Don't let this discourage you. You have to start somewhere, but as you go you will get better. Keep practicing!