How does carpooling work?

Ok, I grew up in the country. Carpooling was riding double on horseback.

Now, I live in the big country. San Antonio has a lot of land. Shoot, one time, after I first moved here, I got turned around on the outter loop and instead going left and being 5 minutes from home, I took a right and made the scenic route of 2 hours (at 70 mph for the most part) around the city.

With gas prices rising and me driving an empty mini van, I've started to ponder "carpooling". How does one find "riders"? Do you take turns? What happens if you run late? What happens if you didn't drive that day but your child is sick at school? How do you run errands? Do you have to pack your lunch or take your riders to lunch with you? Is it permissable to blast rock-n-rock at 7 am to wake yourself up? Oh heck no, I'm not listening to country music for anyone. Are there tax breaks?

I'm only 8 miles from the office and everything I could possibly want is between (or very near) work and home. So would I carpool home, turn around and drive 7 miles back to get groceries?

I feel guilty driving a large empty (except for me) vehicle but I just don't get how carpooling works.