I'm building my own letterhead with LaTeX.

I would have:

• a first page with header and footer
• from second page ahead, just header, but different from first page.

I've already built a one page letterhead with fancy class (I have a header and a footer with images and text)

But, what I'm unable to do is to change the footer and header starting from the second page of the document.

So, I think I should understood when first page end, and the use a new header formatting. Isn't it?

How can I do that?

What you should do do is to define a header and footer for the document and re-define the header and footer for the first page. Here's a snippet from a .cls files I wrote a while ago. Some of the variables, spacings, etc are specific to my use but it should give you a rough idea of what to do.

\pagestyle{fancy}
\fancyfoot{}
\fancyfoot[C]{\thepage}