Welcome to the forum, swatipatra.
Your question is too general. If you have not written any articles, or promotional articles in this case, then it is hard to help you or to provide answers on how you can improve.
Start writing one or two articles just to get yourself started and understand where is your current level in terms of writing. Compare it with all the other related articles on the Internet to get an idea of where you stand. From there, you can keep writing to improve yourself and ask questions that are specific enough over here so that we can help you out.
If you are looking for tips on how to improve or how to get started, head over to Copyblogger. The blog offers a ton of great tips for writers.