My co-worker is trying to get me FIRED!!!?

I work with this woman who is considered a "bully" at work. Everyone is afraid of her because she is mean and she is known for getting people fired. Well, she tried that on me and I put her in her place and ever since then, she has been after me. I work 3rd shift security with only one other person and I rarely talk to anyone because I am there to do a job. I've been there for over a year and I haven't missed a day. During the summer, I come in everyday and clean and prepare the pool (I work at a hotel). Basically now she is running to my boss and telling her I am saying all of these things about her that are not true. Now she is trying to pull the "gay" card. Saying I was talking about her ual orientation. I could care less if she was gay, I just want her to stop talking about me and slandering my name. I had to sign a written warning because she somehow bullied a couple of employees into signing a statement saying I said something. I signed it, but I put my own statement that I was signing to acknowledge the meeting, but I do not agree with the statements because they are not true. How should I deal with this situation? Should I contact HR? I work my *** off for this company and now I have to deal with this drama when in all reality, I never did anything to anyone! Please help...Thanks!