If, like me, your credentials as a woodsman come primarily from watching movies set in the woods, you might think you can just swing any ax down the center of a log to split it. That's not quite right. Like many things in life, you need the right tool for the job.
In this video from This Old House, landscape contractor Roger Cook demonstrates why a normal ax isn't really the best tool to split logs. The narrow ax head, designed for cutting fibres, will probably just get stuck in the wood. What you actually need is a steel wedge and a sledge hammer, or better yet, a maul or splitting ax.
A good old wedge is simple and effective but laborious; if you have a lot of wood to get through, it's best to spring for a modern splitting ax. (Perhaps that's obvious if you grew up chopping wood, but my city upbringing has left me ignorant of ax minutiae.)
Then you want to prop the wood up off the ground, on a larger stump for example, so that you have more impact against the hard surface and better aim so you don't accidentally hit your feet or legs. Watch the video for a few more tips about splitting logs and the proper tools you'll need to prep before winter.
How to Split Logs for Firewood [This Old House]