I bought a new Samsung Galaxy S2 and insert my Prepaid Singtel Sim card. When i turn it on, it doesn't have any network and states 'no service'
Summing up from the comments:
Your device is most likely sim-locked by your provider (even more likely if you bought it there for a discount, or got it free with your contract). So it will only work with SIM cards from that specific provider. If you want to use it with a different (pre-paid) card, you need to have your device unlocked first.
While it is generally possible to do that yourself, it will not only cost you time to figure out how, but you might also risk "making a mess" of it (as t0mm13b put it). So best is to go to some shop offering this service, and pay a little fee having them doing the job. Saves you at least some headaches.
Did you add the APN settings? They are not always automatically generated. Your carrier should provide instructions on how to add /check your APN settings.