Why does my iPhone 4 die at 50%?

I got this phone this last Christmas. These past two weeks it started to die at around 15%. Then it started to stay at 100% most of the day and die at around 30% for a couple of days. Now today, it did the samething but died at 50%. The battery life is ok, but I need to know how to fix it to get it to tell me the right percentage of my battery!! Please help!!