Find an online journal that focuses on a topic that you are knowledgeable about. A quick Internet search will bring up many. You can also consult the website's "Online Writing Jobs" database in the "Journal Writing" category for online writing journals in need of writers. Select a an online journal you would like to write something for, read the submission guidelines and determine if the work is for you.
Write your article to the specifications of the website. Journal writing for an online journal is similar to writing for a print journal in that you need to research and cite your research. Online journals tend to offer a broader spectrum of writing styles than print journals. Get familiar with the guidelines.
Write with a scholarly tone unless guidelines for the online journal state differently. Journal writing is typically written by experts in a field. If you aren't an expert, you should be able to write as if you were, again, only using facts that can be supported. Unless you're writing an op-ed piece or rant, unsupported opinions are not welcome in journal writing.
Think newsworthy. Journal articles typically include the latest research or developments in the field you're writing about. Because online journals adhere to journal standards in this regard, you want current news, but because online journals can be around for so long, you want to integrate information that will be interesting in the long-term scheme as well.