How do I find out how much data I have used?

Settings > Cellular will show how much you have used since you last "Reset Statistics" (scroll all the way to the bottom of that window). You could set a reminder to remind you to do that at the end of your billing cycle each month. Another option would be to see if you cellular carrier has an app (Verizon and AT&T both do for example) that will show you your usage. Another option is the website of your cellular carrier.

What cellular provider do you use? Verizon has a notifications widget that shows me exactly how much I've used during the current billing period.