Set aside a quiet place and time. This place might be in your house, but if your house is chaotic, take a laptop to a coffee shop and sit in a quiet corner. J.K. Rowling started out writing in a local cafe. Do what works for you. Make sure you have what you need (a cup of water, for instance) so you don't have to get up. Let the phone keep ringing on silent mode.
Glue your bottom to a chair. Set a timer for an hour or so, and don't stop until it rings. Unless you have an emergency or you need to take a stretch break, don't get up. Let family and friends know that this is your time to write and you are not to be disturbed.
Write, write, write. Start writing about anything -- the tree outside your window or your eccentric Uncle Bob. This will "warm up" your writing muscles, like stretching before running.
Use different tools to help you get started, including: Keeping a daily journal that you can mine for ideas; using a story-starter that gives you one scenario to write about; describing your hero's personal characteristics; outlining a plot and conflict; and reading the kind of books you want to write for inspiration and to learn.
Allow yourself to make mistakes. Write quickly, to get your thoughts down, then come back later to fix the typos and grammar. Expect to delete a lot of what you write -- this is just part of the process of getting your story on paper.