Where do I start?

[as96]

Is there a guide/documentation on this library?

[Kai]

Yes. Just google for "<X> tutorial/guide", where <X> is one of:
- OpenGL
- OpenAL
- OpenCL
- GLFW
Please understand that LWJGL is merely an enabling technology to use these native libraries with a JVM language such as Java.

There are also people writing/recording tutorials/guides to use OpenGL and GLFW with LWJGL 3.
For this just google "LWJGL3 tutorial".
But understand that LWJGL merely gives access to the underlying native libraries. So you will find much more resources when you google for tutorials/guides for those libraries directly, though those are mostly written for C/C++.

[spasi]

What Kai said is true, there's not much to learn about LWJGL compared to the bindings themselves. But there is a bit of learning required on how LWJGL maps native code concepts to Java code. The LWJGL wiki contains some important information, especially the Bindings FAQ. Note that it's a bit outdated (e.g. on structs), but I plan to update everything in the wiki before the final 3.0 release.

LWJGL also has very rich javadoc.