Depending on the age and use of the phone, the battery might need to get replaced by Apple or an Apple Authorized Service Provider.
To make sure that there are no background activities draining your battery, force restart the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose data by doing this.
Also check the battery Usage and Standby times, they should not be equal. Have a look at the apps that are listed as using most of the battery life as well in Settings/Battery.
Apple Support can check your battery health online, if you contact them: iPhone - Contact Support - Apple Support