Physical exercise 3-5 days a week (30-60 min. per day) is all it will take to notice a great lift in mood, better sleep, and even increased energy and confidence.
It only gets better the harder you push yourself. When you can run a mile without stopping, run even further (and a little harder) next time, eventually move on to 2 miles, then 3, etc. The important thing is that you're breathing hard and feel exhausted by the end of each exercise, that's how you know you've trained well. The same is true of biking, swimming, etc.
Your positive energy will grow stronger as you train harder. It's very difficult for most people to train continuously if they're alone, that's why it's so, so important to have a friend with you to keep you motivated.
It takes several weeks of hard training before you truly notice the changes, in most cases.
No matter how much I'll say this, most people will never have the motivation to actually try it. If they could though, their motivation and energy levels would skyrocket after just a few weeks of proper training. It's almost like a catch-22.