Where can I find statistics on download amount on OSX?
Activity Monitor will show your network usage for the current session, but this gets reset when you reboot. However, if you're just looking for a rough estimate of how much data you use before you get your mobile service, this may suffice. After you get your service, you will probably find that your carrier has a way of checking via a website.

Regarding bmike's answer, I believe he's talking about iOS.