Right now I am migrating to Amazon S3 from standard server. In current server, I have to set structure of directories in order to stay more efficient. For example, current structure of directory is: category_id/article_id/year/month/day/article_image.jpg and so on.
So, do I need to create such structure in the bucket of Amazon S3. Does it affect speed of requests?
This depends a lot on your needs. The path itself is not important for S3, but you may prefer to find all images used in a specific category, on a specific date etc. So I would approach this from your management side, not from the performance side.
That said, if you have a lot of requests (like more than 50 requests per second), you might think of another way to organize your folder structure. Here is a posting on the AWS blog that gives some examples to get the best performance out of S3.