Denis is right on the money - the first culprit is the battery... not all batteries are created equal, since it is a box filled with chemicals, the purity of the ingridients really matters... most proper size batteries will last you a year or two... better batteries will last 3-4 years... excellent batteries can pull a 10 year streak, as the OE batteries made somewhere in europe...

the secondary culprit, that will enhance the problems with the battery, is the worn alternator/voltage regulator... since the voltage output is compromized, even the best of the batteries will remain undercharged, which will significantly cut its usable life...

start with the battery, with a good warranty, this will buy you time to contemplate repair/replace of the alternator...