A problem with my job?

well i found out at mt job that i have been using the wrong on (puting one meal under 1 when it should have been 2 ) i generaly though it was supose to be on that on but the till has been coming up short �30+ on weekends....some of this was my fault. even the manager to the coffe shop i work at told the people over and over to be careful now i feel really bad even though its a part time job and pays not the best. what should i do?