How can i start value investing? Is it better to use SGD 1000 to buy a stock eg. Apple, or would it be better to diversify into S&P 500 using SGD 1000?

If you talked about value investing and think about apple, you are on the wrong track. Apple is already fully developed and most lilely overvalued. In fact, most big companies are.

Value investing is buying black horse. Underdogs. That people dont notice its value.

Value investing is simply buying undervalue firm and one could do that is project the firm future cash flow and discount back to the present. If the price per share is higher than the market price, you should buy the stock.

Broadly speaking, one should always have a diversified portfolio. This would mean owning different asset classes like equity, bonds, commodities etc. A diversified portfolio can also be in a form of geographical location. Whether you use $1000 to buy apple shares or S&P 500, it really depends on your investment objective. No right or wrong perspective.