Processed Foods = Obesity and/or Cancer

My research is about how processed foods are connected with obesity and cancer. It will go through the dangers of the processed foods and how having a bunch of chemicals and manufactured ingredients can have an effect on your body. Through my research, I hope to find the unhealthy damage that processed foods/ingredients can start to cause on your body. I feel that my research falls short in many places, as I catch myself not knowing what to say next. I must say that I need to find more credible sources so I could cite them in my work. That's where most of my trouble is, finding credible sources because it's hard to tell when something sounds credible, but then it has a typo. Which I guess shouldn't make it not credible just because of a typo, but credible pieces don't really include typos. So, that's where I am having trouble. I am involving a bit of history of processed foods, and then including Proposition 65. Summarizing, Prop 65 is basically requiring restaurants to state that some, if not all, has chemicals that has been found to result in cancer and birth defects. I hope to include more information of how it all affects the body, and go into the biology of it. Keeping that tied in with the politics of food will be tricky, so I might end up not doing it. A question I have is, How do I know if an article is credible?