Before posting an article you should go through the guidelines given in the section regarding posting an article. You being a new member, first understand the site well bu going through all the points in the help section. Once you do that you will have a good idea about the site. What are the various sections? Where can you start? What are the rules and regulations for this site. Go through the forum section, ask expert section and read some of the articles already published. This exercise will give you a fairly good idea about the sections where you can contribute. If you have any doubt you can post it in this section as a thread.
The reasons for rejections might have been given by the Editor in the email they have sent you and also at the top of the article in red.
Start your journey. All the best to you.