Just riffing off Perisher’s post a bit:
The STI is an index that tracks the performance of 30 selected Singaporean stocks. You can’t invest in an index directly, but what you can invest in is an exchange-traded fund (ETF) that tracks the STI by buying those 30 stocks in the right proportions. You end up with about the same performance as the STI, plus you get dividends as well (the STI doesn’t think about dividends).
The two active ETFs that track the STI are, as Perisher mentioned, G3B and ES3. You can buy them through your regular brokerage, or, if you’ve got a POSB Invest-Saver account, you can buy G3B through that. They’re basically identical; either one is fine.
The lot size of both of those is about $300-ish, but the good thing about POSB Invest-Saver is that you can buy less than $300 at a time - you can buy as little as one share at a time, with regular, no-fuss debiting from your bank account, so you don’t even need to think about it.