What can I do when git push fails with ..."local out of date"?

I'm sure this is a pretty easy fix, but I'm not sure why my pushes to GitHub are failing.

Right now it looks like:

$ git push origin master
To git@github.com:user_name/project_name.git
 ! [rejected]        master -> master (non-fast-forward)
error: failed to push some refs to 'git@github.com:user_name/project_name.git'
To prevent you from losing history, non-fast-forward updates were rejected
Merge the remote changes before pushing again.  See the 'Note about
fast-forwards' section of 'git push --help' for details.

And:

$ git 开发者_如何转开发remote show origin
* remote origin
  Fetch URL: git@github.com:user_name/project_name.git
  Push  URL: git@github.com:user_name/project_name.git
  HEAD branch: master
  Remote branch:
    master tracked
  Local ref configured for 'git push':
    master pushes to master (local out of date)

Also, when I try a git pull:

$ git pull
You asked me to pull without telling me which branch you
want to merge with, and 'branch.master.merge' in
your configuration file does not tell me, either. Please
specify which branch you want to use on the command line and
try again (e.g. 'git pull <repository> <refspec>').
See git-pull(1) for details.

If you often merge with the same branch, you may want to
use something like the following in your configuration file:

    [branch "master"]
    remote = <nickname>
    merge = <remote-ref>

    [remote "<nickname>"]
    url = <url>
    fetch = <refspec>

See git-config(1) for details.

What did I do to cause this state ... and what should I do to fix it?

To pull from remote

git pull origin master