Walk before you run. Start slowly and build up your activity level gradually. Excitement about a new exercise plan can lead to overdoing it and possibly even injury.
Most healthy adults should get at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous aerobic activity each week. You can mix the two types. Examples of moderate aerobic activity include brisk walking, biking or swimming. Vigorous aerobic activity can include running or swimming laps.
Aim to exercise most days of the week. For even more benefits, and to help keep or speed up weight loss, get 300 minutes or more of moderate aerobic activity each week. Also, aim to do strength training exercises for all major muscle groups at least two times a week.
Do what you love. Almost any form of exercise or movement can increase your fitness level while decreasing your stress. The most important thing is to pick an activity that you enjoy. For example, you might try walking, stair climbing, jogging, dancing, bicycling, yoga, tai chi, gardening, weightlifting or swimming.
And remember, you don't need to join a gym to get moving. Take a walk with the dog, try body-weight exercises or do a yoga video at home.