According to Mayella, in her own words, this is what happened:
“I said come here, nigger, and bust up this chiffarobe for me, I gotta nickel for you. He coulda done it easy enough, he could. So he come in the yard an‘ I went in the house to get him the nickel and I turned around an ’fore I knew it he was on me. Just run up behind me, he did. He got me round the neck, cussin‘ me an’ sayin‘ dirt—I fought’n’hollered, but he had me round the neck. He hit me agin an‘ agin—”
So essentially, she claims that Tom was asked to come over to break a piece of furniture into firewood. Then, she claims he followed her into the house in a very sneaky way and got "on her". She claims that he beat her, grabbed her around the neck, cussed at her and fought with her. She claims that she tried to get him off but he just kept hitting and hitting her.
Chapter 19 will give you the opportunity to hear the story from Tom's point of view. It will be obviously much different.