Good science writing is concise yet thorough, flowing logically from one section to another. It uses an active voice, avoids wordiness, and can follow a number a different formatting guidelines. Click here to find a brief guide to scientific writing developed by MIT.
Generally you'll be writing one of two types of papers: a review article or a primary research article. If you're writing for a class it'll usually be a review article--primary research articles refer to those that you've done an original experimental research project for.
Click here for a science writing cheat sheet that'll give you some ideas about how to get started.