As your child begins to grow and play with other children the difficult art of sharing will begin to come into play. Most children don't understand what sharing is; they feel that if they're allowed to play with it, it's theirs. That's normally when they'll shake their head and yell “mine!”, so here are a few tips to help get your child to share.
Be a good role-model for your child - If your child sees you sharing your time and things with others, than they will begin to mimic and follow your example.
Talk it out - Ask your child how they feel when it comes to sharing a toy, or a cookie, or anything at all. Maybe they're afraid that they won't get a turn or that they may not get it back. When children are young, they don't really understand what they might be feeling or how to name that emotion, so ask questions to try to get to the truth of the matter.
Be where the kids are - If the children are playing on the floor, try to sit on the floor with them and help them to take turns, such as asking the child that is playing with a toy that the other may want if “Sally” can play with the toy once he/she is finished with it. Or if there are two of the same toys, let the child make the decision on which one he/she would like to share.
Encourage it, don't force it - Sometimes children simply do not want to share. If this happens and someone gets upset, try to distract him or her with another toy.
If there's more than one, plan for it - If you have lots of one toy, the chances of sharing are greater because there is enough for two or more. Try playing with a bucket of cars, or in the sandbox with multiples of the same toy. This way, the children will likely share without even knowing it.
Also, keep in mind that children become attached to special items and expect to not have to share this. Have respect for your child and other children by trying to not push the issue of sharing that special toy. Children not only have to learn to share, but they also have to learn to have respect for other's things. Try explaining it to them if they don't understand. “Sally, that's Sarah's teddy and she really loves it very much and doesn't like anyone else to play with it. It means a lot to her.” Then maybe ask the child if they have a special toy that they love very much and explain that it's the same thing with “Sarah's” toy. It will be easier for the child to understand.
As your child gets older, it will be easier to talk about sharing as they learn to talk about their feelings. Keep in mind though that sometimes when kids fight over toys, you just have to stand aside and try to let them work it out. Of course, always step in before somebody gets hurt, but children need to learn how to develop their problem solving skills, and this is a situation of problem solving.
Children will begin to share more as they learn and grow and eventually it will come naturally, but do be prepared for bumps along the road.