First, you need to find a good-sized log. After cutting it into a blank you mount it on the lathe and rough it out. Slowly, you shape it until it vaguely resembles a bowl. On your first try, anything that looks like a bowl is an accomplishment. With practice, you will be able to complete a bowl in a short afternoon by reducing sanding time and learning to hog out the inside quickly.
They also make great gifts, and luckily they're practically free to make. You just have to find a log!
WARNING: woodturning is addicting. You see, the spinning chunks of wood create a vortex that sucks money into it. You may be tempted to spend hours in the workshop, neglecting to eat or do homework.
With all that said, eating and doing homework are not nearly as fun as woodturning :)
And you can find deals on craigslist, too.