Starting any exercise plan, even for a person used to working out, may seem simple on the surface, but there are some pitfalls you need to avoid.
Enter your workout each day with a plan, and always, above all else, listen to your body. It will tell you when you're overdoing it, and it will tell you if you aren't doing enough.
Relying solely upon a weightlifting book or some cardio plan may not be the smartest decision. Each person is different, so give yourself some flexibility and listen closely to how your body feels and reacts.