Choose a basic tribal design, either online or in a book, and have your artist scale it down to fit on the back of your neck. Some tribal images come with a definition or meaning behind them, so whenever possible, seek the particular design that suits your personality, but it is not a necessity as no one will know what it means anyway. It is often best just to choose the design you think is the most visually appealing.
For animal tribal, select a particular animal and instruct your artist to design a tribal image of it. You should also specify whether you want a jagged and pointy tribal design or a more rounded and smooth one. Because the area on the back of your neck is somewhat limited, an entire animal may not be feasible so consider just the head or bust of the animal or only its paw print.
Choose a single word, such as your last name or a word that you like, and instruct your artist to either develop a unique tribal script for the word or use a generic one. There are several types of tribal fonts to choose from, but you should strongly consider having your artist develop one that no one else has. You can try to fit more than one word on the back of your neck, but it is not advised, as it will more than likely appear jumbled and difficult to read.
The easiest way to ensure no one else has the same tattoo as you is to design it yourself. If you have an affinity for tribal designs, consult several examples of them, either online or in books, and draw your very own on a piece of white paper. You can take the drawing to any tattoo artist, who will then translate it into a stencil and ultimately a tattoo, or if you can't draw very well, just sit with your artist and instruct him as he draws it.