1. Have a strong lead. Like any article, you want to start your sports article out with a strong lead, one that encapsulates the available information on "who, what, where, when, why and how." Look at sports articles in almost any paper to see how they introduce the game or event they are covering with their lead paragraph. They get the reader's attention with a strong but concise summary of the story to follow. Also, note that a lead always places emphasis on an important or interesting aspect of the story. For instance, a specific Boston Celtics loss may, in itself, not be as significant or interesting as the fact it is the fifteenth loss the team experienced in a row. The article would tell the story of the game, but the lead would introduce the game with the most important or interesting fact about it, that it's continuing a horrible streak, and would expand on that fact in the article.
2. Write clearly and concisely. If you've ever read Sports Illustrated, you know that some sports articles can be what you might call "literary non-fiction:" lengthy, poetic, filled with metaphor and digressions into back story. If your particular assignment requires that kind of writing, go for it. But if you read the daily sports section of your city paper, you will also notice that most sports articles reporting on the sporting events of the past day are concisely written. Yes, sports articles include context and metaphor and technical sports terms, but they're also to-the-point and generally stick to basic vocabulary.
3. Know the context. Like any news article, a sports article will require you to have a basic working knowledge of the universe on which you're reporting. This may mean not only knowing all about the current players, coaches and standings but knowing some history, as well. This may be common knowledge to you, but if not, you may need to do some research. You will also need to know about the sport itself