My main recommendation, and the one diet change which has made a major improvement in my life, is simply - drink heaps of water. It should be plain water and filtered, I just have a Britta jug and I change the filter every 6 weeks so I'm getting the goodness of water without all the chemicals and rubbish coming from the tap. By drinking at least 2 litres of water a day (I drink 3 litres +) you help your body flush out toxins, keep you healthy, and hydrate your brain which improves concentration, memory, and all that good stuff.
Avoid soft drinks (Coke, Sprite, etc), they're two kinds of bad for your body - they fill your bloodstream with sugar and they're carbonated so they rob the body of oxygen, both things which make it harder for your body to fight off illness. Be careful with fruit juice too, a lot of it is mostly added sugar.
This is an absolutely enormous topic, and I've done a fair bit of research on it over the years, but instead of trying to explain it all here I'll point you to some good resources that really helped me, and also tell you what I personally eat and drink based on all this research. Good diet does actually make a big difference and I find I'm very healthy on my current diet, have heaps of energy, and very rarely get sick.
First resources, I recommend:
The Worlds Healthiest Foods 100 Quick and Easy Healthy Foods Lessons from the Miracle Doctors - This free pdf book is out there and should be taken with a grain of salt, but it's eye-opening and definately worth reading, it also gives the answers to the kinds of questions you've asked. Normally this book is available from
http://www.jonbarron.org but the site seems to be down at the moment so I've uploaded it to my webspace for you.
And this is what I eat on a day-to-day basis, it doesn't change much and I find I'm very healthy, stay thin, and have heaps of energy eating this way.
Breakfast - Two weetbix with soy milk and honey. Cup of tea.
Morning tea - Raisin toast with olive spread (margerine alternative).
Lunch - Tomato soup with brown bread
OR boiled potato with tinned tuna and some brocolli + lemon and salt
OR a big bowl of brocolli, zuchinni, carrots and mushrooms steamed with water, soy sauce and a bit of butter.
Afternoon tea - Cup of tea or coffee and a pear.
Dinner - Baked chicken and vegetables
OR salmon mornay with brown rice
OR roast lamb and vegetables
OR a tomato and mushroom omellette (free range eggs).
Before bed - Rice Puffs (cereal) with honey and soy milk.
I hope that helps
