The first step to take, is to get your diet right. Depends on the fitness routine though. Say you want to do more cardio enhanced exercises, you should eat more fibre rich foods like spagetti whole grains etc and proteins like fish and chicken. Your diet is very important. Your exercise should not exceed 3 hours a week in the beginning. So if you work out for an half hour a day, by going for a jog, swim, cycle for 6 days and continue to stick to both you diet and your exercise routine, you should get fitter and so increase your exercise routine by an hour a week or so. Just dont overdo it the first month!