Choose a topic that is well established in the discipline, but only weakly represented on Wikipedia. The best choice is a topic where a lot of literature is available but isn't covered extensively on Wikipedia.
Gravitate toward "stub" and "start" class articles. These articles have only 1-2 paragraphs of information and are in need of expansion. Relevant WikiProject pages can provide a list of stubs that need improvement.
Before creating a new article, spend 15-20 minutes searching related topics on Wikipedia to make sure your topic isn't already covered. Often, an article may already exist under another name or as a subsection of a broader article.