If you don't have a data plan, they will charge you. So every time you go onto the internet and load a page on the browser or download something using the internet, they will charge you. I think right now it is at 5 cents a MB, but in your area it may be different amount.
Additional Info: A data plan is something you add to your phone in addition to the talk(minutes) plan you have. If you want to be able to use the internet and not get charged per KB/MB then you purchase a data plan to go along with the talk plan. Also, Texting is not with the data plan, it is it's own charge. So, really what you can get for a phone is The minutes you use every month, and if you wanted to use the internet without getting charged per every use, you buy a data plan that can be around $20 a month, then if you like to text A lot, you get a texting plan.
Data is anything you use that is on the internet. If you use your phone to go to a website to view anything or if you use it to download email or even check your email. That is using data. The only way you will not be charge for that is, if your phone has Wifi(wireless internet) and you used your own wireless connection or you went to some store that offered free WIFI and you use it. Or you have a data plan that you purchased when the phone was turned on.
You can call them up, go into the store or check your account online. as long as you have all the necessary information they need to be able to check your account.
Hope this extra info helped you. Good luck!