To increase your serotonin levels naturally try changing your diet so that you increase your bodies serotonin levels by eating healthy foods that give you energy such as fruits, vegetables and any other foods that contain Vitamin C and other nutrients.
Other things you can do besides changing your diet is to get plenty of sleep, get plenty of sunlight and get plenty of exercise in the sun as well.
In the winter try using a depression light to help boost your energy and Serotonin levels.
You should also try avoiding and lowering your stress levels as well as take a Vitamin Supplement such as Vitamin D or Vitamin C supplements to help boost your mood.
You can also take some fish oil supplements that help to improve your brain health as well and eat fish when possible.
Fish is known as the brain food so you'll have an improved memory when eating fish and your serotonin levels should naturally increase.