The first thing you need is an idea. You cannot just sit down at a computer and hope the story comes to you. Find a story through your life experience or even from the news itself.
Many writers once they have an idea benefit from creating a loose outline of the story. You can put chapter headings into this one as well as characters. This helps you keep track of the story and where you want it to go.
There are authors who write their novels long-hand and those who use word-processing programs. Use whichever feels the most comfortable and natural to you. Just remember that if you write it out long-hand it will eventually have to be typed.
Every writer at some point reaches a point where he runs into writer's block. Just keep writing. You can even jump ahead to a different part of the story. Or you can have characters within the story talk about your writer's block or plot points and work them out that way. Then you can go back and fix the story during a rewrite.
For the first draft, keep writing. Don't spend too much time worrying about punctuation or plot points you feel don't work. Get the story down on paper.
Find an editor or reread the story yourself and do at least one rewrite before you try submitting to publishers. No story is usually ready for publication after just one pass.