Why is junk food unhealthy and if I want to eat it what's kind should I choose chicken burger beef burger. .......?

Veggie burgers, . Chicken, turkey and once in a while you should have the real thing!
It's the fat & salt. Junk food is unhealthy because it's laden with fat, salt and simple carbohydrates. In your example, if you want a burger, look for grilled chicken, turkey or even veggie burgers. And don't top it with cheese or bacon.