How to make my own mod on minecraft?

In single player, add it to your mods folder and play.
In multiplayer, set up your own server. Mods only work online when the server has been set up to allow for it.

Learn Java

Once you've learned Java then knowing how to make the mod and implement it into the game should be very easy. There isn't a step by step tutorial on making a mod, but you can learn Java here - http://www.learnjavaonline.org/