With just $100/month, what can I invest in? I know $100 is not a lot, and I might incur more fees, but it’s all I can afford right now… Any suggestions?

If you have only $100/m to spare, it is best to just invest in knowledge such as books and online courses.

None of the investment options mentioned below will give you any meaningful return. Worst still, by investing $100/m in RSP, it gives you an illustration that you have done something. This will lead to procrastination to invest more in the future.

Firstly, glad to hear you investing! :)
I recently got into investing & am thankful for this local platform to educate Singaporeans such as ourselves

I believe after reading the comments of the others I would just echo/overlap some of theirs and leave the final decision to you

See if you have any debts to clear of /things you have to pay & settle them as soon as possible
Then start with this 40-30-20-10 principle: 40% expenses (unless you live with your parents then hahahaah guess you will spend lesser than expected) 30% investment 20% savings (CPF! https://blog.seedly.sg/about-central-provident-...) & 10% insurance

You have the liberty to override this rule of thumb until you have an emergency fund (6-12 months total expenses for your livelihood on a rainy day)
You also have the liberty to adjust the ratio 40-30-20-10 to your current situation ☺️ (becus I do myself)
You may want to consider Roboadvisors eg Stashaway (using it myself) because of the minimum investment

I would end of by encouraging you to do risk assessment (DBS/PSB provide free assessment if you have an acct) so you know where you stand & enjoy the process of exploring more as you visit this AMAZING site

All the best for your endeavors!