The answers to these questions can be found through a close reading of chapter 19. In this chapter, Atticus questions Tom Robinson on the stand, and he gets a chance to tell his side of the story. Mayella asked him to get something off of the dresser, and so he gets up on the chair. While he is on the chair, she grabs him around the legs; this throws him off balance, and he hops down, knocking the chair over. Then, Tom states, "she sorta jumped on me." So, she jumped on him, and hugged him around the waist, then kissed him on the side of the face. He tried to run out, but she grabbed him, so he had to push her away. At that point, Mayella's dad came in, and Tom ran away.
That is not the story that Mayella and her father tell, however. Atticus, through his questioning, makes it pretty clear that they are lying and that Tom is telling the truth. I hope that helped; good luck!