How can I stop my battery from overcharging?

I have a Peugot 106 XS (1995 M)

There was a weird smell coming through my vents and when I lifted the bonney, my battery had blown a seal and the liquid within was boiling and therefore spraying all over the inside of the car.

I've been told that this could be due to my battery overcharging or because of an earthing strap.

Has anybody had this problem before and is there a way I can fix this myself without having to shell out lots of cash?