This all depends on your theme. Typically the theme's code will use the post title as an H1 on the post page and an H2 on the home, category, archives, and tags pages.
Use all-in-one seopack and do not worry about that h1s... If you want you can change that manualy in pages and css files... I do that sometimes when I need perfections couse I want to put h1 as a title for "post page"... but to be h2 on main and category page...if you have knowledge of css and can find out in your code that is not so hard...
What do you mean exactly? Are you talking about how to change the appearance of the <h1> and <h2> tags or how to assign these tags to text in a post? Chances are good that you can find the current <h1> and <h2> in your style.css file in the /wp-content/themes/yourthemename directory.
you can usually find it at the main index page, view the editor or the source code of your page. go to your main page, just right click anywhere then click view page source, you will be able to view the code of your website and click ctrl+f then type in H1 or H2 to automatically locate it.
I think you can just put the tag and it will automatically generated. But it will be depending on the theme that you have if it is allowed.
you can find it under themes and try to search on the codes or you can take the source code to view those tags.
What I want is to change the font size if the title in my blog (in the signature), how can I do that? I need to find it and edit the code, but where to looks for?