You start at the beginning.

Welcome to 3FC.
The first step is choosing to do something about it. You have to make up your mind that you don't want live like you have been. This is all about you. Find a plan that works for you and stick with it. The reason you don't stay with anything long is because it's either not working or not sustainable. You have to find something that is both. There are number of plans around here that folks use and they all work for the most part for some folks. Every plan doesn't work for every person. It'll take patience, time, and dedication. It will not happen overnight.
Find the time to get to the gym, or walk around the block if you don't have time. Find a way. Get moving.
Get rid of the junk food. Change the way you shop.
There is some support around here for folks that work shift too. You might look into that as well.