You might want to first try a different charger cord to rule that out as the problem.
If that doesn't help, you just might need a new battery. If that is the case, then first you need to know if you are under warranty still or not. If you are, you don't want to open it up, but should instead contact the manufacturer about a repair or replacement.
If it isn't under warranty anymore, then you can change it out yourself (there are videos online that can walk you through it), or take it to someone to do it for you.