Welcome to Android Central! Did you get the SIM from Straight Talk? In order to use the phone on Straight Talk, they would have had to give you the SIM card, or if it's a CDMA phone, you would have had to register it with them. Do you remember what you had to do?
Also, what plan do you have? Their cheapest plan allows 100 MB of data per month, so if you're using it up quickly, then you might not have any more mobile data access for the rest of the month.