First you'll need to create a requirements document that outlines all the features of your game, in detail (leave no stone unturned). For estimating, prepare a work breakdown structure of all your tasks and take a best guess of how long you think it will take you. You should also include learning tasks. If you're new to WebGL and HTML5, you will want to add a task to learn those two in detail. Or you may chose to use an existing framework, so you'll need to add a task to learn that as well.
My advice would be to start much smaller. Do something you can complete in a much shorter timespan with far less requirements. As you start to build up your experience and your codebase, you can start to take on much more demanding projects.