You need to find out what is using the power. There are many possibilities, so no single answer.
Install a battery monitoring app such as GSam Battery Monitor, leave it running for most of a charge cycle, then see what is using the power. Look at which apps use most, look also at "time held awake" by each app.
Also, what's your signal like? If you spend most of your time in weak signal areas, or where the phone spends a lot of time changing signal bands (dropping from 3G to 2G, or LTE to 3G) then that also uses a lot of power.
Also, it depends on what you mean by "really quick" - if you mean that it's flat in 6 hours of sitting in your pocket, that's wrong. If you mean that 4 hours playing games exhausts it, that's to be expected.