Check the battery report under settings on your phone. If you are using an Android phone and found out that your Phone Idle consumes a huge amount of battery then something or some app is running on your phone even when it's idle that drains your battery. I have recommended app to combat this, I use "Greenify" try to search it on google playstore, and watch the video on how to configure it. Once you get the hang of it, you'll be able to use it to its full potential
How long have you had your iPhone? If it's an older one it's likely that it may be time to replace your battery. If you have a newer one then do as the previous poster suggested and check what's using your battery. If you truly feel like the battery isn't holding a charge as it should and you're still under warranty (1 year manufacturers, 2 years if you have AppleCare+) contact Apple. If they deem your battery defective they will replace it for free. If you have insurance you can opt to use it but sometimes it's cheaper to pay for a battery replacement versus paying the deductible.