Often we forget about what we're putting into our mouths, because we get hungry, have cravings and often need to grab something to eat fast.
If you can spend a little time to learn new habits it will be rewarding. By spending time, I mean do a little research on alternative foods and ways to cook. The first time you go to the shop to buy something new, look at what is made from and where it was made. The next time you go back you will already know to just grab the better item from the shelf.
Not sure about switching from cheap vegetable oil for your cooking? Well bigger supermarkets now have alternatives that will make you feel so much better - look for rice bran oil, coconut oil or ghee.
Realised your margarine or spread contains a lot of things you don't like - think about returning to plain butter - and watch out for those that say they're spreadable butter but add more than just cream to make it soft.
Here is an interesting blog on the subject.