I am assuming that you want to post someones timeline, you can do that if he/she is your friends and he/she has allowed friends to post in his/her timeline.
1. Type the person's name in the search bar at the top of your page.
2. Click the box that says Write something to... and choose what type of story you want to share (ex: Post, Photo/Video)
3. You can also: Tag friends. Pick a date for the story. Add a photo. ...
4. Click Post.
Let me know if looking something different.