There are a number of ways, I feel, start writing a book (first is an idea of the type of book)
1. Think of how you want it to end. That gives you something solid to work towards. A kind of goal, if you will.
2. List possible ups and downs, confrontations, obstacles the characters may face. Don't have to use them all but it sometimes helps with character development and what not
3. Or Let's start at the beginning, a perfectly good place to start. Start with what type of conflict. Man vs man, man vs nature, man vs himself, man vs society, man vs technology. First decide on that, once you get the idea of what the main conflict is (mind you, there can be more than just the one but I feel you should have one major conflict). Develop the main character (loosely for you may find as you write the character may change ever so slightly or drastically), keep the main conflict in mind, but just start writing and see where it takes You! Sorry this was long winded and I hope it makes sense