Hi Bharath,
As Jun suggested, please go through the API first where you can find it by yourself from next time! 
However for this time, I would give you the answer. Use 'selectionFinish' event where you can fetch selected Items of MultiComboBox.
API: JsDoc Report - SAP UI development Toolkit for HTML5 - API Reference - sap.m.MultiComboBox
Regards,
Sai Vellanki.