It has to be someone you can trust and preferably someone you have worked with before. Because a Startup's main chance of success comes from how well the team functions and communicates with each other and trusts each other, a cto should be also someone who is very knowledgeable and has good networking relations. Every person in a startup is a sales or marketing person, so keep that in mind.
This roles takes me to answering your question: go network. Your CTO should be a networker and thus be found while networking. A lot of Startups are created by friends and positions assigned accordingly, but when is just you and your building your team networking is best & more direct solution. There are rules for networking properly, but that's a different topic.
Give me a call if you would like to go over networking tips and strstegies.

Best of luck!
Humberto Valle
Unthink Strategy