I’m a 23 year old male and recently got interested in investing. However, I have only made one investment so far in POSB’s Nikko AM STI ETF at $100 a month. Any advice on the next step?

Especially agree on the first liner by Jacky!

1. Reading also means by learning!

Since you are interested in investing, you will be far easier to pick up investing compare to those aren't.

2. Build up Investing Capital

Investing need capital, hence, find more ways to build up your capital, by decreasing expense, or increasing income.
3. Learn about your risk profile

Jacky is far more risk-seeking person, hence he goes on to investment with higher risk. However, it may not suit everyone, especially if one couldn't take the huge fluctuation of tech stocks.
Hence, if you are a risk-averse person, you may start with ETF, REITs, where the risk and price fluctuation will comparably lower.

4. Take care downside

Before invest, do have your risk protected. Have a good coverage of insurance and emergency funds. So, you wouldn't need to liquidate your investment if the situation arises.
5. Diversification

There is a lot of way of diversification, but I especially like simple ones, that doesn't complicate. I followed the three-bucket strategy. Personal Bucket - which is mentioned under point 4 (To take care downside). The 2nd bucket is Market Bucket- where invest in ETF or companies that could at least hedge interest. 3rd bucket is aspiration bucket - where you grow your pot thru Growth Companies like tech stocks.
Hope it helps!

I'd suggest that you go for a robo-advisory platform to do the job of assessing your current financial position and recommend a portfolio strategy after reviewing your risk profile. As for the "catch", I would say that Robo-advisors are still not very different from your ordinary financial advisors as both options will still have a management fee incurred for users. The difference lies with the amount, as Robo-advisors have lower management fees.
I work at kristal.AI, and my mojo is to help people make the right financial decisions. If you think I helped you, do give me "Thumbs up". If you think my response was biased let me know, I will work on it.

I hope this helps you make the right decision.

Firstly, kudos on taking the first step in investing! You can look into dollar-cost averaging (DCA) every few months through a regular savings plan (https://blog.seedly.sg/which-regular-savings-pl...).

By doing DCA at a specific date each period, you can take advantage of market falls without emotions affecting you. Investing is a long-term plan so one must not be put off by volatility in the stock market.

As a first step, you should determine your risk profile though given at a young age your risk taking ability is likely higher, I would suggest evaluate a balanced Kristal like All Weather / All Rounder / Steady Growth based on your risk profile. Or just take any balanced portfolio of US Equities (SPY), EM Equities (VWO/EEM) and Gold (GLD) clubbed with Bond ETFs (try UCITS preferably LQDA or a BND, LQD)

Also do run the algorithm at kristal.AI (investments upto $50,000 are free) and see what that shows for your profile. Happy to help out if you need more info.