Be sure to eat one to two hours before your workout. You need the calories to provide the energy for a good workout session. A snack or light meal is good.
Have a plan for your workout. Decide what you are going to do before you start. Not having a plan makes it easy to skip or forget parts of your program leaving you with a less than optimal workout session.
Always warm up at the beginning. No matter which type of training you start with, cardio, strength or flexibility, warm up is essential. Muscles, ligaments and joints need to be brought to an exercising state.
Always cool down. Don't leave your exercise session sweating and breathing hard. Slow down and cool down first. Bring your body back to a non-exercising state.
Drink water. Hydrate yourself adequately, especially before and after your workout. Lack of energy is one of the signs of dehydration. Your cells need water to function well.