Well, how old is your phone. If it reached its 2nd birthday, no wonder that the battery is slowly dying. Although if the phones battery is brand new and you suspect that it depletes faster THAN it should then maybe try uninstalling some apps, maybe even do a factory reset, if that doesnt help, well I guess you could try flashing a brand new rom, although, it's a bit difficult, but not impossible. Especially when you have a known brand (samsung) there should be a lot more guides that could help