how to open library in the onshape website??????

What do you mean "Library"?
Sounds like you need to complete some of the Learning Pathways. They are the best place to start and will save you tons of time.

.

If you are referring to all public documents, then those can be found here on your main page:

If you are referring to the Standard Parts Library, then that is located from within your assemblies. You have to create a document → open an assembly tab → Insert → standard content

.