First, . Read the Abstract and skim the Introduction and Discussion. This will tell you what the authors intended to prove, how they went about it, and what actually happened.
Next, read the Methods and Materials section and the Results. These are the most complex parts of the article, but from them you will learn the details of the authors' methodology and the results of their research.
Third, re-read and take notes.
If you can answer the following questions about the article, then you have a pretty good understanding of it.