Lots of us carry a small note book in our pocket to record our punch in/start work time and punch out time for every day we work. The following week we check our pay check against our time books to make sure they are paying us properly. If you are not doing so, start doing it now and get into the habit early. As for the hours you have been getting screwed/mistakenly not paid properly for, keep writing it down every week that you are short and continue to ask your supervisor(nicely ask for now) to fix the shortage. Once you have made seniority you will be in a better position to demand they fix it. They can wait until the following weeks paycheck to fix shortages, below a certain amount, based on if you are part time, which you are, if you are full time(check the contract or ask your shop steward for one if you have not received one yet to get the $ dollar amounts). However, once the total amount owed is above that threshold you are entitled to penalty pay if they do not correct the shortage within the contractually agreed time frame(this is why you will want to get a contact book) so you know the rules you are working under and what rules the company must follow.
Yes I do have one and no I will not look up or site the language/article for you, since this will give you the incentive to get the contract book!