To start with VGA, you need to understand how a screen/monitor/CRT works. You will need to get yourself familiar with the such parameters as horizontal and vertical timings, screen refresh rate/pixel rate, screen resolution, etc.
A very basic VGA requires 3 bit input for the three basic colors i.e. red, green and blue, a combination of these three colors give you 8 different colors. Another two inputs for the VGA screen are the horizontal and vertical syncs, which deals with the horizontal and vertical timings according to the refresh rates, etc.
There are many books and online resources available. I would suggest is the a book FPGA prototyping with Verilog, it has VGA covered in good details.