Advise on stamina building for biking.

Back in my youth I was really active and did many sports including mountain biking which I still enjoy the most now. Over the past 6 years I turned into a slob playing video games and eating junk all the time. This year I've decided to get my act together as I've become unfit and lost a lot of confidence in myself. I've researched healthy diets but nothing seems to work for me and I find it fairly confusing. I was just wondering if anyone could share some advice on how to build stamina for biking and also how to eat properly?