Sharing is a very important social skill at the basis for many interactions with others. At the beginning of Kindergarten, some children haven't had the opportunities to learn and practice this skill while others are quite fluent with the concept.
I begin the lesson with a story of Marley eating all of the candy he got while Macy got a sad look on her face and went into the other room. Most of the students understand and infer that Macy was sad because Marley didn't share! We discuss why sharing with friends is important; its nice, makes friends feel happy, makes ourselves feel happy, etch. We also discuss there are many items that are not safe to share with others (I do make exceptions for example sharing an ice cream cone with mom or dad which a few kids said they do).
We read and discuss It's Mine by Leo Lionni. Then I use colored versions of the pictures from the worksheet below for a large group discussions of thing to share or things not safe to share.
The lesson was adapted from Puzzle Pieces; Classroom Guidance Connection by Gwen M. Sitsch & Diane S. Senn and Small Group Counseling for Children, Grades K-2, by Diane S. Senn.