Get writing and don't stop until you have your novel or several short stories. Then go back through, edit and proofread. Make whatever it is you produce sound and look its absolute best.
Learn as much as you can about the writers market. Your local library likely has some contacts and journals for you to look at. These will help you get in touch with publishing agents as well as publishing houses.
Make sure your work is in the correct format. Publishing companies will vary on this. Usually, typed, double space and error free is the best way to go.
Send a cover letter along with your work. The letter will vary according to the type of work that you submit. Short stories need a brief introduction, while novels will sometimes require a chapter-by-chapter outline.
Keep track of all your submissions and who you send them to. If you write a lot then create a spread sheet. This will help keep everything organized.
Learn to handle rejection. It will happen. Just keep writing and keep trying. You will get that publishing deal sooner or later.