First off - congratulations on choosing nursing as a career! You'll probably meet many negative attitudes about nursing on your way through school (I know I did), but ignore them. Nursing is great.
I'm a new grad myself - I graduated 7 months ago and have been working ona medical unit since, but I'll be starting my first labour and delivery position in January.
The first thing you have to do to become an OB nurse is get though nursing school. It's a lot of studying, and hard work, and coffee, and TEARS, but nursing school is an experience of a lifetime, so soak it up.
If you get a chance, or have any free time, try to volunteer in a maternity unit. Maybe feeding babies in the nursery, or even stocking carts in labour and delivery. It will look good on your resume, you'll learn some new termilogy, and most importantly, staff around the unit will know your name and your face.
Try to get to any maternity conferences/inservices/teaching programs that you can. Some nursing schools will sponsor students to get extra certifications. Make sure you put them on your resume. It'll show how keen and dedicated you are.
And then when you graduate, apply for any maternal/child position that comes up. Even if you're not qualified for the position, applying will show your interest. And even if it's not the specific area of maternity you want, it's a foot in the door.
Be patient too. Some hospitals have a lot of competition for maternal/child positions so it may take a while.