This depends on the status and age of your partners. Students would always use "du" with each other. But if you address someone who is older than you or you don't know his/her status you should use "Sie".

When talking to someone informally on the internet, "Du" is the common way to address the other person. In real life, if both person's age is below 20 years, they normally use "Du" to address each other, even they don't know each other. "Sie" is for people in a position higher than yours. Your teacher, your boss at work. It's also for grown-ups who do not know each other. For example you talking to a salesperson in a shop. Or random person on the street when asking for directions. Take care when watching movies. In the German versions of Hollywood movies the actors sometimes use "Sie" even when a "Du" would be more appropriate (never happens the other way though).