I am a using 3G Internet and I manage my data transfer, using 3G WatchDog. There is inbuilt settings available for managing your data usage in Andorid, but still I like this App. It has full control, you can set cycle limit(monthly, weekly, daily) and it alerts you when it reaches the specified % of your data plan.And you can define your data plan maximum Limit. It also shows which App used how much of the data. That will help you to use your apps wisely.
Here is the screenshot of my 3G watchDog.
It also shows your WiFi usage. And it shows the Apps data usage under WiFi.