A Book of Shadows is a spiritual journal kept by many solitary and traditional Witches. It is a collection of information designed to build reference and spiritual growth, for yourself or for a coven. It is also used in many covens to teach new Witches about the craft.
What goes into a Book of Shadows, is up to the individual or coven. I would advise if you are starting in the craft, to start one as soon as you can. Put everything in it. You can put anything slightly relating to magick in your BOS: Passages from your favorite books, information you have gathered on magickal topics, words of wisdom, Insights, magickal poems, dream iterpertation, divination results, listing of pagan stores, order houses, and contacts, and most importantly Spells and Rituals that you have used or designed.
I recomend the first thing you put into you book of shadows is the Wiccan Rede