If that's the only argument you have, you can use $* in your script to get all the parameters. Also, quote them:
function push() {
git add --all
git commit -m "$*"
git push
}
You can use "$1", too, leaving place for more parameters, but in that case you must use quotes while invoking the function (say, $ push "Some message" instead of just $ push Some message). It's more correct, but for this function purposes - just to economise time - it may be more useful.
Anyway, I don't think it's a good idea to have such a function. git add is a good moment to review your commit, decide what to include in it and what to leave for other commits, and to construct a really nice commit message. Plus, you would often want to delay the push until you have a couple of commits.
I recommend you to use something like tig for easily selecting what to stage and the like.