If you want to have a great online sample, it should be a mid-length, SEO, well-edited sample. Create an article that is keyword focused and about 400 words long. Keep every part of the article focused on the topic and cut out any fluff or asides to keep the article packed with information as well as succinct. I would prepare at least two such articles on different topics. Most of the content sites that I've worked with have required two writing samples. Once you have written the samples, edit them well, leaving no typos and making sure that they flow well. Good luck!