There may be a subtle difference. You can write "papers" that are not for publication in a periodical journal. Example: students at any level may be asked to write a paper on something, but it's almost never called an article unless it appears in print in a periodical of some sort (which can even be a school journal). But when you say "article", it generally refers to a body of text that is published as part of a periodical. In the academic context, the term generally refers to something that appears in a peer-reviewed journal.
If it's understood that you're speaking only in the context of peer-reviewed literature, "article" and "paper" may be used interchangeably. But if the context is ambiguous, then "paper" has a wider meaning.
As for the second question, there are many generic guides on the internet to help you in writing your first scientific article. However, it's most important that you read the specific instructions for the journal that you are submitting your work to. That can usually be found online under "Instructions for Authors".