VHDL
VHDL is the language that GHDL understands. If you have mastered it good for you and if you have not, here you can find some recommendations on how to learn it.
The Developing Environment
The tutorials section allows you to get started in setting up the right environment to begin doing your VHDL coding. Any GNU/Linux environment is good. Windows or Apple OS X is also supported by GHDL.
Books and Online Material
Among the many books, tutorials and free material, it is recommended that you begin with one of the many VHDL books you can find on Amazon or elsewhere. Alternatively, any free material or VHDL tutorial will be a good start.