How do I take care of a hostile employer?

I work at a coffee shop, and many things have happened there that were very hostile and abusive towards me. I would like to know what I can do to solve this problem. For example, a lady came through the drive-through the other day, asking for decaf. My co-worker told her it will take about 5 minutes to make because we make decaf fresh to order on 3rd shift. She said, "That's ok. I don't mind waiting." So my friend gives her the total tells her to pull forward. And then another car pulls through. So I leave the back of the store to come up front and help her with the orders. I start making the next cars food when my co-worker came up to me and said, "Will you please look in my register and tell this lady I don't have any $20 bills." So I walked over to the register checked it and we lifted the drawer and showed her and I said, "Ma'am there's clearly no $20's in our drawer. I'm sorry." and I walked back to making my order as the woman in the passenger seat starts yelling,"Well let me tell you what! You were rude!" At this point my co-worker just walked away from the window to avoid getting anything started. So the woman who was yelling at us came in the store cursing and continuing to yell at us, while i'm trying to finish the other order. My co-worker who just got back in town only a few hours before continues to talk to the woman at the car who is arguing with her. I go over to hand the rest of the order as the woman who is inside is still yelling and cursing at us. All I can hear my co-worker say is,"Ma'am, all I said was it was going to take 5 minutes to make" and "there is no 20." So I walk over to the woman who is still yelling and cursing at us inside the store trying to figure out what she's yelling about. And the only thing I can hear her say is, "theiving f-ing n-er", "You should both lose your f-ing jobs," and "Neither one of you should have an f-ing job," all this along with other profanities. While she was cursing at us and calling us names. I snapped. I told her to get out of the store. I told her she was rude. I was talking to my supervisor on the phone when it happened. I told him how she was behaving towards us. And he told me to call the police, because she was being hostile and we don't have to take that kind of treatment. So I told her to get out of the store. Just leave the store. And she kept yelling, "I don't have to if I don't want to!" So I said, "Just leave I'm calling the police." And she replied with, "Good! I hope you do!" I told my supervisor I had to call the police and I told my other employee to get the license plate number and when I said that the woman ran out of the store. The police came and filed a report about the situation. We showed him the registers and he verified there was no $20 and that we clearly didn't steal her money. He took the womans information along with her license plate number, car color and make, and the direction she was heading. He gave us a business card with his information on the matter. My co-worker told me she was offended because she felt that the woman was accusing her of stealing because she was black. I was upset because that's what I collected from the womans accusations, so we just left it at what it was after the officer left and went on working. The next day I'm getting ready to leave from work my manager calls me into his office. He presented me with a letter the woman had wrote stating she came through and was talked to rudely so she tried to come in the store to wave me over and I started yelling at her and my african-american co-worker. The letter said if I have any other problems I am to be permanently terminated. My manager told me I had to sign before I left. I told him that letter wasn't the complete truth. More than half of it was a lie. And he said,"When you come at people bitching they think your the asshole." I told him I didn't come at her bitching. She came in the store in a hostile manner cursing at us and yelling accusing us of stealing from her. She called my co-worker a "theiving f-ing n-er" and it made me mad. So yes I told her to get out. But we called the police and he went to look for the woman. My manager didn't care he said I had to sign the letter before I left. So I initialed it, because I thought I was going to lose my job if I didn't. And today she wrote another letter and my manager posted it on the office door to show everyone in the store. This is just one of the many incidents that have occurred at this store. Other problems that arose are, one of my other managers threatened me with a pair of scissors to my eyes, and he also threw a knife at me. I was waiting on my boyfriend to pick me up one day. And I was upset because he was late. It had been 30min. and I was worried and my manager threw a knife at me and said,"Here you should cut your wrist and then kill him." There are cameras in the store. Yet anytime something happens and I try to solve the matters with my managers they take the other persons side and accuse me for being at fault. I can't take this abusive, hostile, threatening treatment anymore. The womans letter is threatening my job. And I know that leads to an Interference with contract/Interference with business relations. I am a recent paralegal graduate. So I know I have rights. I need help from a lawyer. The only reason I'm working this job is because I'm still searching for a paralegal job. However, things have gotten way out of hand. I have many other situations that I've written down. I got what I can remember in dates, and like I said there are cameras in the store. Can anybody else tell me what else I can do?