Battery draining quickly in iPhone I have an iPhone SE running iOS 12 and the battery is running out quickly, 

How can I found out what is draining the battery and how can it be prevented?
To see battery usage by app go to Settings -> Battery and you can see the battery usage by  app for last 24 hours of last 10 days. Then you can see what is using the battery.

About the battery usage on your iPhone, iPad, and iPod touch

You can click battery health to check the battery health to see if it needs replacing.

Alternatively you can try updating to the latest software. Later software updates improve battery life performance.

From this Apple Support article:


  
  Plug your device into power and connect to the Internet with Wi-Fi.
  Tap Settings > General > Software Update.
  Tap Download and Install. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or
  Cancel. Later, iOS will reinstall apps that it removed.
  To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just plug your
  iOS device into power before you go to sleep. Your device will update
  automatically overnight.
  


If your phone is low in battery you can preserve is by turning on low power mode in battery settings which turn off some features.

Use Low Power Mode to save battery life on your iPhone