This article helps you to create a new libraries in EAGLE .lbr format. Mostly the inbuilt library of the EAGLE will be mostly enough for you to make a new design. However, sometimes you might need to design your own device package, symbol and then link both of them to create the element.
Although this information is there in Autodesk’s website, you can google it easily, I have made a pdf of the entire process of the three steps. You can download these pdfs below:
Part 1 PDF click here
Part 2 PDF click here
Part 3 PDF click here
The articles are pretty simple and very well written and explained along with pictures. Still if you need any help comment below.