Ask is important. Always reach out to people who you believe could be great mentors to you and be specific in your ask for example, don't just ask "please be my Mentor" be more specific then that as to what help you are looking for, expectations, time commitments etc.

There are specific organizations who you can reach out for a Mentor match as well, for example, if you're a young entrepreneur in Canada, you could reach out to CYBF Canadian Youth Business Foundation www.cybf.ca or in US you could go for SCORE http://www.score.org/mentors and many more.