For 2D:Use Canvas,Easy And Based On WebGL 2
For 3D:Use Three.js Or Whitestorm.js Or Hilo3d
Notes:I Learned Making HTML5 Games From Mozilla Developers Network Website And W3Schools In Graphics Courses List!!!
At first, you need to have an idea of what will be game about. After that you can do every game type (but it depends on your skills and sometimes you will need to learn another web technologies, too - PHP, SQL etc.).