Welcome aboard!
Since I'm the first to reply I get to state the most obvious and easy answers.
Diet and exercise are 2 key components to a healthy lifestyle.
Your current state of health and your age is a big factor in how you approach your eating habits and how much you should be exercising.
By state of health I mean:
Are you over weight or under weight?
How much exercise to you currently get?
Do you have any health issues that influence your diet and/or ability to exercise? ( Diabetes, heart trouble, high blood pressure, bad back, etc.)
Do a self evaluation to see what you think your current state of health is and then decide where you need to improve.
A person who is 30 years old and 10 lbs over weight, who doesn't get much exercise but is in good general health can be a bit more aggressive in making a lifestyle change than a person that's over 50 years old with a history of heart trouble.
I'm going to let some of the other folks take it from here. Good luck to you!