How do you push a girl against a wall and kiss her without hurting her?

When you push her, put your hands on her shoulders. Shove her forward with force, but not so hard that her head clunks against the wall. In bed, alternate between hickeys and nips. Tell her that you want her to let you know if you need to bite harder at all. Scratching her back lightly with your nails would probably also be beneficial to her. Try pinning her arms when you kiss as well.