You need a new one. 10-12 years is now considered the average life-span for a waterheater. If yours is over 10 years and possibly close to 15 it should be replaced. Your tank is leaking on the bottom (one of the most common places that you find the leak on an aged water heater as well). Your tank is cracked or fractured. Repairing a 10-15 year old tank could cost as much as a new one.

We just replaced ours last year... Our unit lasted almost 20 freakin years (how I don't know, but according to the date installed it was nearly 20 years ago). They're not making them like that anymore though.