Different markets require different amounts of capital to day trade. Stocks are popular, but also the most capital intensive. If you want to day trade stocks in the US, the absolute minimum you need is $25,000. And you'll actually need more because you need to keep your balance above $25,000. Starting with $30,000 or more is recommended. The stock market provides up to 4:1 leverage on day trades.
Assess how much capital you're willing to risk on each trade. Many successful day traders risk less than 1% to 2% of their account per trade. If you have a $40,000 trading account and are willing to risk 0.5% of your capital on each trade, your maximum loss per trade is $200 (0.005 x $40,000). Set aside a surplus amount of funds you can trade with and you're prepared to lose. Remember, it may or may not happen.