Is it possible to lose weight just by cutting out junk food?

I mean you should ideally nourish your body with vitamins, etc. in fruits, veggies, etc., but there's nothing inherently bad about pizza. If you can eat it and stay within your calories for the day you will lose weight.

Sort of. It's mostly calories in/calories out, but not all calories are created equal. 100g of chocolate has more calories, less nutritional value and will keep you satisfied for a shorter period of time than 100g of, say, tuna will, in addition to causing a much more sudden rise in your blood sugar. So yes, if you're eliminating those junk foods and replacing them weight-for-weight with foods that are less energy-dense, you will lose weight (and your body will probably function better). @DrEnalg is right, it is possible to overeat healthy food, but I suspect that if you're mostly consuming junk that the risk of that is low. Plus you're unlikely to eat 1425g of celery isntead of 36g of Hershey's Kisses (each is 200cal) - check out this chart http://imgur.com/gallery/MeEpOni

Completely cutting out junk works for some people and not for others, and cutting out entire food groups can be difficult to maintain as a lifestyle and cause you to miss out on nutrients - for example, if you're going out for dinner it can be harder to avoid "junk" food, so you can either not go out for dinner (nope) or just make better, more educated choices. Generally, "everything in moderation" is a good guide... but you have to be aware of what "moderation" means. Start by logging every bite of what you would normally eat for a week or so, junk and all, just so that you're aware of what you're actually eating. And then educate yourself on what you should be fuelling your body with. A lot of people seem to find this post helpful http://community.myfitnesspal.com/en/discussion/1080242/a-guide-to-get-you-started-on-your-path-to-sexypants/p1

Good luck!