Drink lots of water
Eat the most unprocessed foods you can find-whole grains, raw or steamed veggies, no and I mean NO refined sugars of any kind, and deep frying stuff is a big no-no.
No alcohol for a while. At least not for like a month so your body can adjust to its new healthy lifestyle.
Get involved with a bunch of people who are into activites... like for example, a cycling club- have fun, be healthy, and EARN those evenings at the pub!

Most important, think POSITIVE. Think of yourself as a healthy person. You will become one a lot sooner!