Im a student age 21. Currently i only earn 200 from my part time job. How can i start to invest?

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I answered a similar question on another thread! I would recommend saving up for your emergency fund first. Assuming you already have that emergency fund in place and any excess is for investments:
With $0: Start investing in your own financial knowledge:

1. Read forums for financial knowledge. SeekingAlpha, MotleyFool, Seedly etc.
2. Read quality news: Financial Times etc (subject to paywall)
3. Read books on both Value Investing and investments in general. Assuming you are based in Singapore, our National Library is a good place to start.

With $100, if you really want to start investing with small sums: You can start investing with as little as $100 with Monthly Investment Plans for a start. Some to look at are:

1. POSB Invest Saver
2. OCBC Blue Chip Investment Plan (BCIP)
3. Maybank Kim Eng Monthly Investment Plan
4. Phillip Share Builder Plan