Get one of these:
You probably need a new battery.
One thing I’d like for both my older vehicles – if wishing could make it true I mean — is a digital volt meter display for the battery voltage. Do most new cars these days include this feature?
edit: The product post by @insightful would work on my truck, but on my Corolla the cigarette lighter is above the cupholders, so it would get in the way of my Big Gulp.