What kind of returns should I be expecting as a beginner investor?

A more direct anwer would be 4-5%, there is a 'risk-free' return from CPF at up to 3.5% for OA and 5% for SA - so ignoring the illquidity of the above, you should be aiming for that. Also if you look at the historical returns of some of the blue chips or the STI index, 4-5% is definitely acheiveable even without deep investing knowledge

If you are investing in SG market, you should aim to beat the STI returns. Because if you cannot beat STI returns, then you know you should just buy STI ETF.

This applies to overseas stocks too. If you invest in US stocks, can you beat the S&P500 or Dow Jones returns?

I think it ultimately depends on your risk appetite. Risk and returns are directly related, when one goes up, the other goes up as well. If your risk appetite is crazy high, penny stocks allow one to double your money in a day, though the inverse is true as well where you can lose half your money in a day. But as a beginner, you must first understand your risk appetite before deciding on what sort of asset classes you would like to pursue in your investment journey

4-5% will be good start for low-mid low risk exposure.