If you don't know what tethered data is and don't know how you used it, call your service provider immediately and remove that feature from your plan.
To check your data usage, some service providers offer an app that you can use to check your near real time data usage statistics as well as call minutes and texts. Check in the app store for your service provider. AT&T has an app and I use it quite a bit, but I also have text alerts set up for when I reach certain milestones in data usage each month.