Toddlers on a whole are generally do not share so well. While a select few toddlers are born generous, most kids do not naturally share until they are no longer toddlers at all being 7 to 8 years old. Before this point sharing must be a learned behavior that you as a parent teach, well at least try to teach and in most cases simply enforce.
The first step in teaching a child to share is to share yourself. When the opportunity presents share with your toddler and explain what sharing is as well as why you are doing it. An example would be, if you have a cookie, ask your toddler if he or she wants some (which naturally will be a yes) pull a piece off and say, “mommy/daddy shares with you so we can both have some. Sharing makes both of us happy.” Or something similar.
When your toddler does share, or sort of share reinforce this behavior by offering praise. In the beginning, your toddler will likely only show people things to share so to speak, like letting another child touch or look at his or her toy, but not actually play with it. Even this type of behavior should be acknowledged, as it is a step in the ladder to sharing.
As try to give your toddler certain items that he or she never has to share, items that are all theirs. This helps toddlers feel less like all of their things are being taken away when they are sharing. In the toy example, perhaps have toys that are for sharing with friends and toys that are only for your child.
Lastly avoid punishing a lack of sharing. You can express disappointment, such as saying, “Oh no! *insert name* is sad now because you won’t share your toys, that’s not very nice.” However, the child should not be placed in time-out or offered any form any discipline. Especially with siblings, you may encounter one child taking items from another. In this situation simply remove the item and return it to the original child, tell the offending child that he or she must wait his/her turn and that taking things from other people is not nice. If the behavior continues, remove the item in question and be sure that your toddler knows the reason no one gets the item in question is that he or she will not share.
With a little effort and time within a few years most children get the hang of sharing just fine.