Working at Mcdonalds. Need Advice.?

I'm an 18 year old recent graduate. I got hired to work at mcdonalds at the beginning of my senior year. I wasnt very well trained, the crew trainer was a guy around my age at the time and there was another trainee who was a year younger (she also complained about the lack of effort on his part). All he would do is flirt, which got very annoying. After my 4 hour/4 day training week i got transfered to the mcdonalds im currently at. At the start it was very difficult i had to learn everything as i went on because when i asked for help the managers would get annoyed one even accused me of being a whinner, which i most certainly am not! So i kept my mouth shut after that, when i made i mistake i learned from it and even though it made them very angry i ignored it. Ive gotten 3 write ups so far, and it upsets me so much. All my co workers have been working there for years, out of the new crew members im the only one whos stuck around for so long. Ive wanted to quit so many times but i dont because the money is very much needed at home. Just last week i graduated on wednesday, on tuesday one of my managers calls me and says "Where are you? Had you any idea your suppost to be working right now?" Never had i been scheduled to work on a tuesday i mostly worked weekends and one or two days of the week. I had never before missed work and when i do work im always 30 mins early (being late irritates me) I told her i must of misread my schedule and apologized. She told me i was getting a write up for not going. i let it go that time and decided not to be so hard on myself. On saturday i was scheduled to work, when i went back on sunday a manager told me i had yet ANOTHER write up for being short 2 dollars and 84 cents. So i got suspended for a week!!!!!!!! I am aware you can only be short 2 dollars but one of my other co workers was over 5 dollars and she didnt get a write up because she got a perfect score on both of our mistery shoppers. I hate how they do this!! i am sick of mcdonalds i feel really stupid working there, can somebody please tell me what i should do?