Here's the best advice I can give you, and having grown up in a family business, I would like to think it's valid advice.
Don't expect to make any money in the first few years. Be happy if you break even and consider it a job well done.
Now, to break that down some: Needless to say this business is expensive... equipment is one obvious cost, but people don't think about other costs:
Time: the time investment is heavy. The more photos you take/gigs you have, the more work you have. The more you want to get gigs, the more you have to spend time selling yourself, networking, spreading the word.
Advertising: Even if it's posting an advert in the local classifieds, that costs money. I live in the middle of nowhere between Buffalo and Rochester in upstate New York. It's nearly $90 dollars for an ad about the size of a large business card, and that's for it to run once in the bridal section that comes out once a year. A basic classified ad with necessary info can cost you about $20 for a day. And for those that think Craigslist is a good venue - yea, sure, if you don't mind your ads getting bundled up with the ones of photographers "looking for Asian models for private photo shoots" and "I will DJ your love-making session" - I wish I was exagerating too...
Rental: Are you going to rent a place to do portraits? Do you need a window to showcase your work and promote yourself to the local public? Will you have to rent a warehouse to do certain shoots? Do you have to rent storage space to keep all your stuff?
More studio "things": I am not into props - in fact I downright detest them because you can either never have enough or you start falling into the prop-trap (always using the handful of props that "work"). So if you're into props, you will be buying buying buying. Don't forget backdrops, filters, lights, flashes, cabling, extension cords, tripods, stands, booms, reflectors, diffusers, etc.
Professional Organizations: It pays to belong to certain organizations, particularly if it helps you get business. For example, the local pro-photographers organization, the national organization, the local business bureau/chamber of commerce, a business coop, etc.
Insurance: In any business you need it. In this business, you even need a disatisfaction insurance - just in case they hate your work and want to sue... yay.
Business plan: there are books that talk about how to do this. Your local small business office has people that can help you do this and even give you sample forms you can use to build yours. Basically it's a short term and long term plan to grow your business. It helps to know where you're going before you get started.
Obviously, the more time you have to dedicate to this job, the more likely you will make more money. Full time or part-time - it can make a difference.
I do this part-time, mostly because my "day-job" pays me a lot as an IT-guy and it offers us medical insurance. For me to quit my day-job and still make , I would have to book at least 50 big package weddings a year at prices my local area can support.
But I'm OK doing it part-time - it fits my schedule and allows me to do what I love and get paid for it. This past year was nice and busy... had a bit over 40 gigs including weddings, senior shoots, portraits, and some commercial work. And even though I bought a K20, rented lenses, upgraded my computer storage and monitors, still made a tiny profit.
Best part of it all is when the tax man cometh, he will owe me money