The first thing you should think about is the size of your average losing trade. Lets say its 40 pips or at $5 per pip, 200 dollars.
Then make sure the $200 loss is no more than 2% of your account balance. On that basis, you'd need 10 grand. Realistically if your new to the game, reducing your risk to 1% you'd need 20 grand, or 0.5% per trade 40 grand etc.
The above assumes that you want to make money, you could probably use 500:1 leverage, and open it with a weeks pocket money, but you'll be broke within an hour !