I found some great exercise guidelines from the American College of Sports Medicine (ACSM).
Basic recommendations from ACSM and AHA (American Heart Association):
Do moderately intense cardio 30 minutes a day, five days a week
Do vigorously intense cardio 20 minutes a day, 3 days a week
Do eight to 10 strength-training exercises, eight to 12 repetitions of each exercise twice a week.
Moderate-intensity physical activity means working hard enough to raise your heart rate and break a sweat, yet still being able to carry on a conversation. It should be noted that to lose weight or maintain weight loss, 60 to 90 minutes of physical activity may be necessary. The 30-minute recommendation is for the average healthy adult to maintain health and reduce the risk for chronic disease.
But can't seem to get things going? Take this Barriers to Exercise Assessment and then read the tips for how to get past them.
Where to start/What to do
Jumping into exercise can be overwhelming when you haven't done it before. Here is a great page to find helpful information, including a pre-exercise risk assessment, tips for overcoming barriers, basic strength training and stretch programs to get you started.