Photography skills aside, and they are important of course, you also need to think about the business aspects.
Pricing. Set your prices too high, you don't get the business. Set your prices too low, you lose money and go out of business. Once you've set your prices, it's hard to raise them. So do your research first. Photographers can be very generous with a new member of the profession, if you aren't undercutting them on price.... which devalues everyone's value.
Are you going to invoice people? Are you going to accept deposits for future work? How are you going to handle situations where your client demands their money back because they expected an Annie Liebovitz style shot, but only had a budget for a 2 hour session?
Copyright. Do you have at least a basic knowledge of who owns the copyright, and what you can do with images after taking them. Do you have a basic knowledge of privacy laws, and who you can photograph, and under what conditions you can or can not "publish" the images?
Do need a business license? If so, are you going to fly under the radar for a bit? Most beginners do - but there are risks - so you know what they are?
Are you going to collect and pay taxes? (sales and income). If not (many beginners don't) do you know what benefits you are giving up? Because there are some benefits not being in the black-market.
Do you need insurance? For loss of your equipment, accidental or otherwise. Liability - in case you knock someone or something over while you have a camera pressed to your face. The most common photographic insurance claim is from wedding/event photographers who have knocked someone/something over while backing up.
You don't need to be an expert in all of this stuff to get started, but the more you know, the better off you will be and the more professional you will appear. And less risk you run bad and expensive things happening to you.
Good Luck.
And yes, budget for an external flash. Shoot in RAW. Budget for getting Lightroom or Aperture. Get business cards made professionally. Budget for buying at least two more lenses and back up camera body in the next year or so. And lots and lots of HDD storage, plus a way to organize your client info and the images.