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A lot of capital, a lot of hard work, proper strategy, a lot of patience and zero emotion.

Lots of capital. So, when your trade goes against you, you can take countermeasures. You also need a lot of capital for buffers. If you can wait long enough, most trades are profitable eventually. If you have big enough capital, you can always turn a losing trade into a profitable one.

Please do check my longer answer below...

For day trading, you ideally need to understand 3 ares well:

  1. Fundamental analysis
  2. Technical analysis
  3. Trading psychology

Every financial asset that you are trading will have specific patterns which you can learn and either skills will give you an advantage.
How do you obtain such knowledge? Start reading sites such as http://investopedia.com

Then you will need to choose a platform for trading the assets you wish to trade e..g. MT4 or regular currencies.

These platforms usually allow you to create a guest account and will give you some virtual money to test your investment skills and platform. e..g virtual demo money for USD$ 5000. Learn the platform and test with the virtual money and trade it with discipline as if you are using real money. If you start bad habits, they can be difficult to lose.

You can learn a lot by playing no-limit holdem the RIGHT way. Bankroll management is so important!

I think you need to invest more and earn less in order to play safe in Day trading while their are more chances of earning passive income from frequent trading.