Hi Vijay,
This is one of the best places to get started with UI5.
https://sapui5.netweaver.ondemand.com/sdk/#content/Overview.html
The developer guide is pretty comprehensive, however I suggest you learn the Javascript fundamentals before looking at this. A good book is Javascript the definitive guide by David Flanagan.
Also this is a good post by Kevin Small to help get you going
Top 10 things ABAP developers should know when learning JavaScript
Regards,
Katan