Depends on the method you are looking to earn money on.
If you're talking about Hubpages, well, it takes creativity, originality, effort(like spider girl says), also time like she says, you need to be willing to expand your knowledge and write about more interesting topics every day. You need to know how to promote things you're selling, not too much, and not too little, and on the right place.
You can probably learn a lot from taking a look at other successful hubs. Try to do the same thing they do, but on your original subjects, if you can.