Hi Yen. Which carrier are you with? Maybe I can help you with the complete instruction for activating your prepaid SIM.
While most places you buy your phone from WON'T set up your new phone for you FREE. Mind you the last one i bought at a Walmart store the fellow did set it up for me FREE. BB and Telus (my carrier) wanted a fee for the 5 min service it would take them. But Telus did tell me how to do it all and I went home and phoned them and got a walk though. I suggest you phone your carrier (sim card company) and ask them