Q: If I use cellular for just a little while in a month and cancel, do they charge me next month even though I canclled for awhile?

I haven't begun ATT cellular service because I don't know what the cost will be for what.  If I use cellular for just a few hours in a month and turn it off, do I forfeit the rest of what I paid for that month?  Will they continue to renew each month even if I don't use it?  When they say "cancel" do they mean "turn off"?  Can I cancel and restart frequently?  I'm on a strict budget and don't operate like the government does.  I pay as I go and don't live beyond my means.  How does the ATT program work?