These are a few ideas for beginning your Book of Shadows. It is by no means a comprehensive list, and there’s no reason for you to include everything here if you don’t want to.
Deity: Information on Gods and Goddesses in general; what each deity represents, as well as more detailed information on the God(s) and Goddess(es) that you feel a particular closeness with.
The Sabbats: Information on when the eight Sabbats are, and what things are associated with them, as well as the story they tell about the cycles of nature. You may wish to include rituals and activities for each specific day.
Esbats: You can also include information on the phases of the moon, and the different types of magic that work best in each phase. It may be helpful to record the dates of new and full moons.
Rituals and Spells: Keep a record of spells and rituals you have tried, and the results of each, as well as any other insights you may have on them. You could also note down any rituals or spells that you find particularly useful, and may wish to try in the future.
As mentioned above, this is by no means a comprehensive list. You could also include any poems, chants or stories that you find particularly meaningful, such as the Charge of the God/dess, or the Wiccan Rede. Some people like to keep a dream journal as part of their Book of Shadows, although you could keep it separately if you prefer. If you have a particular interest in some form of divination, you could write down information on the Tarot, the runes, etc., and record your thoughts and feelings about it in general, or on specific readings. You can also add in your own personal insights, your thoughts on life, Paganism and anything you feel is relevant.
Remember that your Book of Shadows is a working book, not something to be written out prettily, and admired on a shelf. Write down your own opinions on everything, especially spells, rituals or other articles that have been written by someone else. It’s your personal book, so make it personal. Date everything you add in, so that even if your views change, you have something to look back on to see how you grew to feel the way you do.