I've been a "CTO". I've known lots of "CTOs".
First bit of advice: change your title to something else.
Among tech entrepreneurs the term "CTO" carries some of the same connotations as "architect" does among enterprise developers: people paid extra to do nothing.
Second: figure out where on the org chart you've just been placed.
Are you the kind of CTO that's intended to do nothing at the top of the engineering organization? That's what you're expect, right? But more CTOs that I've known were instead expected to do nothing at the top of the marketing organization; in particular, to serve as the "spiritual head" of the product management team. The reason for this: the "CTO" is inherently customer-facing (impressing customers with pronouncements from the CTO is one of only two benefits obtainable by your company by naming a "CTO", the other being "retaining you even after your coworkers decide they want to remove your commit privileges").
Third: is your company big enough to have a C-anything?
Even if you (smartly) rechristen yourself "director/research" or "director/product" or "director/engineering", if you're working at a 4 person company, you can make yourself look dumb --- not just at your company, which is like all companies likely doomed, but on your resume, where you'll flip the bozo bit on people evaluating you for their team by allowing yourself to be labeled with a grandiose title.
The reality is that in small startups, particularly pre-revenue startups, and particularly when your employers have conceded to you a title ostensibly meant to communicate some authority, you can probably do anything you want. Pick one thing, do it extraordinarily well (clue: you are not doing it extraordinarily well until it hurts to keep doing it that well), and keep doing it. Things a "CTO" (please don't call yourself that) can do:
* While maintaining a role in the peloton of committers on your team†, become the arbitrator of all tech controversies (not the decider, the arbitrator).
* While maintaining that committer role, build the engineering team by recruiting talent (note: this only works if you recruit people who turn out to be awesome for the business) and retaining it.
* Become the marketing face of the company, which can work if your company's audience is nerds (or if you sell direct to other companies, but you don't appear to be doing that).
* Become the head of product management, which means you spend virtually all your time engaged with customers (either directly or by measurement and testing) and almost zero time building.
† DO NOT GIVE UP COMMIT. Corrollary: you must keep committing, meaningfully, or it will become easy to take commit from you.