I got a new phone and I need to tell it that I've already used 25 megabytes this period. With the stock app I can I can set a usage limit, but I can't tell it how much I've already used this month.
Go to Settings > Data usage, and you'll see a graph showing your data usage over a certain period. The coloured bars can be adjusted to set a warning, and also a limit, to stop you using too much data. Further down the page, you'll also see a list of your apps, and how much data they use.
You can also download alternative data monitoring apps https://play.google.com/store/apps/details?id=com.mobidia.android.mdm