Assuming you are talking about general fatigue and not a specific medical condition:
1. Get enough sleep. This is number one for a reason. Most people need around eight hours of sleep per night, although some people may need more or less. If you're not getting enough sleep, your body will be tired during the day no matter what else you do.
2. Eat healthy foods. Eating nutritious foods helps your body to have the energy it needs to function well during the day. Make sure to include plenty of fruits, vegetables, and whole grains in your diet, and limit sugary and fatty foods.
3. Exercise regularly. Exercise releases endorphins that can boost your mood and energy levels, so even if you don't feel like working out, it's still worth it in the long run. A moderate amount of exercise is best - too much can actually lead to fatigue instead of reducing it.
4. Take breaks throughout the day. When you're feeling fatigued, take a few minutes to yourself to rest and rejuvenate. Take a short walk outside, meditate for a few minutes, or just close your eyes and breathe deeply for a few minutes - anything that will help you relax and recharge your batteries.