A common question I hear is "Why is my iPhone battery draining so quickly?". There many different factors and reasons for this and there is no single easy answer. However, there are a simple thing you can do to help figure out what the issue might be.
Background Apps Running
The first (and best) thing to do is check the battery usage screen that Apple provides.
Open the Settings app.
Scroll down a little ways and find the Battery setting.
Tap it and wait for the screen to load completely. It will take 10-30 secs (depending on your type of phone) for the app to show the battery usages stats. Once it is open you will see a list of apps and the percentage of battery that it used in the last 24 hours sorted from highest usage to lowest. This list should give you a good indication as to what is draining your battery.
For more details, tap the little clock icon to switch the display to show how much time the app used the battery. It also shows details like screen time and background time.
Age of Phone
Keep in mind, an iPhone that is more then 2 years old will probably start to show signs of quick battery drain. You may start to notice that your battery jumps from 30% to 5% in the matter of minutes or after you open an app. At this point, your only option is to have the battery replaced.
You can try to replace it yourself. There are a few sites that show you how to do this and allow you to buy the new battery. I like iFixit's website. Or, you can have Apple replace it.