What does this iPhone message mean?

My iPhone is kind of new and this is the first time I've gotten this message from AT&T:

AT&T Free Msg: This is a courtesy reminder that you have used 90% of your 3GB data plan allowance for the bill cycle ending on 08/06/13. Any overage is charged @ $10/1GB. Learn more at www.att.com/dataplans. Tip: Mobile Data is unlimited over Wi-Fi

How do I stop using data?

Anything that would otherwise use Wi-Fi used data. Any apps like Facebook, Twitter, Safari, etc will use your data. The only way to not use it, is to not use apps and Internet.

It means that if you don't start using Wi-Fi more often youre going to surpass your data limit and once you pass it theyll charge 10 dollars for every gig of data you go over. Doesn't matter what phone you have if you have att or verizon youll get a data limit.

Well getting apps and videos uses most of your data so stop buying that and you'll save data. What it means is that AT&T is warning you that you have used 90% of your data and you have 10% left. It also is telling you that if you want to get more data it $10 a GB.

Connect to the Wi-Fi at your home and If you are out in public don't use the Internet unless you can find Wi-Fi until the month ends also I you want to know how much of your data you used call *3282# and AT&T will send you a message