As long as your friends meet the following criteria, it'll be fine. Although I strongly recommend asking someone like a former teacher or colleague to be your referee as they would be in a better position to attest to your work ethic.
- Known for at least
1 year
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Cannot be your family members or anyone who is blood related to you
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Cannot be a NSF (currently serving national service)
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Must Be working in a full time occupation which is contributing to CPF
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Must Be at least 21 years of age
When you apply through the Careers@GOV portal for the first time, you'll be asked to create an online resume and its good enough (for this job at least).
You can provide a cover letter if you want but it's not a must. If you want to know what to write, you can google on some writing guides.
All the best!