I haven't done online dating, being married and all lol, but I support scorpiosis37's approach. A couple back and forth messages to see if you connect beyond profiles, then in person sooner.

As for the guy you want to cancel with... "I need to cancel with you. I realized I jumped into this online dating thing before I was ready. Wishing you all the best." The end. If he replies with anything other than an acknowledgement type message, do not respond. There is no need to continue a dialogue.