The main thing is to use utf-8 encoding. That covers the biggest character set. You can include this in your web pages by placing this after the <head> tag:
<meta http-equiv="content-type" content="text/html; charset=utf-8">
but you also need to ensure that your server is sending out the pages with uft-8 encoding, as this overrides the on-page encoding.
You can also specify a language for each page, such as
<html <font color='"#FF0000"'>lang="hy"</font>>
which is supposed to help browsers and search engines deal with the page appropriately.
You can just use a normal doctype of your choice.
In terms of fonts, it probably doesn't matter, as long as you use something that's common on Armenian computers and that has the required characters. (Most fonts seem to have a wide character set these dyas, but I'm not familiar with the Armenian alphabet.)