Don't overdo your workouts or rush into an exercise routine that is too intense for you. This is a recipe for disaster and you'll end up injuring yourself. Start out slow and work your way up to more intense workouts. As you get stronger, increase how much you do to see the best results.
You should change your fitness routine periodically so that you continue to benefit from exercising. If you use the same routine for too long your body becomes used to it and the benefits start to decrease. Adding a new exercise can also be fun and keep you from getting bored with your workouts.