Diet, weight lifting, and conditioning are the three areas to concentrate on...
Start by researching good diets and start tracking your eating habits in a journal. It will be easier to make adjustments if you know what you are already eating. Don't just guess because mostly everybody under or over estimates what they truely consume.
Start a conditioning program. This can be as simple as walking a couple of times a day to doing HIT conditioning routines. Being in better condition will help increase your metabolism and help you recover better from workouts.
Research proper lifting routines, but please look into basic and abreviated routines. Three days a week full body routines are a fantastic way to start. Take a couple of months to workout light, get proper form dialed-in, and condition your body to the new demands. This also gives you time to increase your conditioning and get your new eating adjusted.