My girl is 5 -year-old and has become too possessive about her things, she doesn't share any of her thing with anyone. How can I inculcate the quality of sharing in her?

You can buy multiple quantities of the same product to start with and then ask for one from her. Also buy few things for yourself which she might like but give them to her only when she shares some of her stuffs. This works! Kids at this tender age tend to imitate what parents do :). So try to analyse yourself from her perspective. She might have learnt it indirectly from you based on how you protect your mobile and laptops ;)