First start by resetting the SMC, heres the Apple TN on it [http://support.apple.com/kb/HT3964|SMC Reset]. See if that solves your battery issue. If not your battery may need to be replaced and you will need to get your system to an Apple Store or get the needed tools and parts to service the system up your self.
Warning! your good intentions doing this your self with little experience could damage your system beyond repair. I would not make this my first repair job.