What I have to do to write and have published an article? Let's suppose that I have proved a theorem myself and I want to write an article about it, I have a few questions:
1) How do I have to write it ? I mean, what character should I use, what conventions I should follow, what format should I use and so on...
2) How do I know that my result is original ? I know that there are dozens of new theorems proven every day so how can I be sure that the article I send is original (maybe who will publish it has to check this) ?
3) To what magazine/site should I send it ? Maybe my result is original but it isn't very interesting so probably Annales of Mathematics isn't interested but are there any magazines/journals which publish mathematic of not too high level ?

I'd say: Write a good introduction to your article and answers to these questions will come naturally.
Since you are supposed to place your result in the already existing literature on the problem, you will:
1) See what notations/conventions/format are used by others to deal with the problem. Use them! People like what they already know (particularly for the notations) :).
2) Once you'll know about the literature on your problem, it should start to be clear how innovative is your result.
3) Where are the references on your problem published? This could at least give you a vague idea on who (which journal) could be interested by your result.
