I ask because I keep hearing young technical cofounders call themselves CTOs, and I think about how much we don't know (I'm fairly young too).
Best of luck!
The best thing I learned from that was, as a CTO you have to "measure everything and control the conversation"
EDIT: got that for ya - http://news.ycombinator.com/item?id=509571
Drilling down, as "CTO" I'd consider myself responsible for making sure of all the following (needless to say, this isn't an exaustive list):
Business continuity: if you're the only one diligent enough to make sure you're backups are happening and work, do it (in the bad old days that meant running the tape backup every day). Never be too proud to sweep the floor.
As CTO, you're responsible for "what": the architecture and consistency of your systems. Visit the Joel Test and do it in reverse.
Depending on the people below you, you can leave a lot of the "how" to them, ideally you'll mostly supervise there. Ideally they'll come to consensus on contentious issues, beware of playing the role of a tiebreaker.
Ah, yes, the people: learn how to recruit well. There's lots of good advice to be found there (e.g. go to the Ask the Headhunter site and use it in reverse). For specific advice, I'll note that one of my best bosses would never hire someone who he had doubts about. In a startup, it's generally the case that a wrong hire is worse than no hire at all.
Sales, marketing, manufacturing (if that's an issue) and technical coherence. Make sure everyone is on the same page. That's one of the things that killed Symbolics, e.g. they spent a lot of money on a factory that couldn't make what engineering was designing, not that marketing was promoting what engineering was designing or the reverse.
Make sure there's a product manager for each product, his job is among other things to help make sure the above is happening.
Good luck!
Founders who give themselves the important buzz word titles is a false positive to productivity. Like joining a gym and not working out. You get instant gratification just from joining.