The error message is essentially saying that someone else pushed a change to github (including you, if you used a different computer), and the branch you're trying to push does not yet include the other person's changes. If you force push, the other person's changes will be lost.
Fetch, then Rebase master onto origin/master (resolve conflicts if there are any), then Push. // This moves your work so that it comes after the other person's work in the commit graph.
Pull and resolve conflicts, then Push (this makes more of a mess out of the git log though) // This merges your commit with the other person's commit and makes a merge node in the commit graph.
You may have to repeat the process if the other person/people push new commits in the meantime.
In order to get the syntax right for each command, you should refer to the git documentation while you do it. I don't personally use the command line because it's fairly easy to accidentally make a typo and do something COMPLETELY different than what you want, so I don't remember the proper command line argument order to use.
Edited by Nypyren, 04 July 2014 - 06:09 PM.