It really depends on your personal situation, but you can open a firm for a very small amount of capital these days. This is fortunate because credit is tight and as a brand new attorney you will probably have trouble getting a traditional business loan.
The best way to figure out how much money you need is to write up a business plan. A business plan also improves your chances of getting a loan, if you need one. It shows the forethought banks like to see.
You need to be honest about your chances of bringing in clients, whether you have good referral sources, marketing opportunities, etc. Also, look at how much money you need to live on and whether you have the necessary resources for that along with the money to keep your practice running. I would suggest you plan for at least 3 months without any money coming in. Could be longer, could be shorter, but 3 months is a good starting point.
You can certainly do as Andrea suggests, but make sure you know at what point you will find yourself in financial trouble due to over use of credit cards. You also need to think about your own level of comfort with risk. I know the job market is terrible right now, so you, like many, might be in a situation where opening your own firm is your best choice, so your aversion or lack thereof to risk might be irrelevant.
These days, opening the office itself, especially if you are using virtual space, is the least of the issues. Technology is relatively inexpensive and you can get most of what you need for low monthly costs. Internet connection, phone, domain name, cloud-based exchange email, secure cloud based document storage (for backup,) simple website for marketing, etc.
Make sure that you are prepared for the ethical aspects of practice in terms of operating/trust accounting, organization so you don’t miss SOLs, client confidentiality, etc. That is where most people get in trouble. So make sure you spend whatever money you need to spend so you are properly organized. It will save you money and headaches later if you set yourself up properly in the first place.
Also, malpractice insurance is expensive, but the sooner you can get it, the better.
Rather then reinvent the wheel, I’ll send you to an article my ex business partner wrote on the subject of creating a business plan.