No doubt you already have an interesting story in mind. What you need to do next is commit your tale to paper. Before you begin it is best to find a comfortable place to work and set up a writing schedule you can adhere to.
I like to lock the front door, unplug the telephone and write, non-stop, for hours. That works for me but, for most, this is impractical. What you should do is set aside a specific amount of time – each day if possible – to work on your book. Try to pick a time when you expect minimal interruptions. Once a daily routine is established, you will make steady progress.
You will need a word processor. The standard is Microsoft Word and, if you can afford it, I recommend you purchase the Microsoft Office Suite. If not, check out free applications such as Open Office.
When I write I keep the Thesaurus.com website open in the background. This site is useful for finding synonyms for words I overuse.
One final point; you should write about what you know. Otherwise your work will be delayed as you painstakingly research your subject. I should know. I spent most of a day pouring over maps in an attempt to plan the route of two of my characters across the United States. In the end I still got it wrong!