You might have seen this chart already, it's from the LA Times. Somebody made this useful (and funny) flowchart to help you figure out whether to 'tu' or to 'vous' someone.
I usually wait for the other person to see what they call me and I use the same word back. If they don't use either as it hasn't come up in the conversation yet, I would tend to use 'vous' UNLESS we are of a similar age or they are younger than me. I've met people younger than me who wanted to 'vous' me and I asked them to 'tu' me and they still 'vous'ed me! I dunno... for me it's kind of like being called Madame instead of Mademoiselle. It makes me feel old!
I had also read that online, people tend to 'tu' each other.
Another interesting thing to do is to watch French films and see whether the characters use vous or tu depending on their relationship with each other.
tutoyer - the act of calling the other person 'tu' (informal 'you')
vousvoyer - the act of calling the other person 'vous' (formal 'you')
tutoiement / vouvoiement - the manner of addressing the ther person
Have fun! ;)