First of all
and good for you for deciding that you want this to be a healthy lifestyle change! This is about getting good exercise and the right kinds of nutrition ^^
I usually suggest baby steps at first. Start with drinking more water and using leaner proteins, more whole grains, fruits & vegetables... things of that nature. You may also want to track your food or calories so that you are aware of how much you are eating. Also, the sooner you start scheduling in exercise the better. Exercise is that something that many people struggle to keep doing consistently, and it's very important. Again, you can start small so it isn't overwhelming! Short 15 minute walks or treadmilling.
Metabolism is a funny thing... and I don't know all that goes into making sure it's at peak performance. I do know that it's important to a functioning body, and that healthier changes in eating and exercising will most likely boost it if it's slacking.
Most people are done growing after they've left college, and they become much more sedentary (not as much walking to classes, or if you played a sport, etc.). Those extra calories aren't used for what they were before, now they just become extra fat. And just because you used to be able to eat all those things doesn't mean that you were any healthier because of it. So don't mourn the loss of those days too much! This is about being healthier overall
You CAN do this!
Think about it and let us all know what you're planning on doing to start with
Maybe more people will have ideas to help. And absolutely keep posting & stopping in for encouragement & support! We all love to know how others are doing!
Best of luck!!