the correct html tag for headings are the h series tags ; so, the largest header (i use it for the title) would be <h1>title</h1>, next largest is h2, etc.
usually i use h3 for chapters but that's a question of choice. in the css code, you can modify the font used, whether it's bold or not, italic, etc.
edit : or see pepak's much better answer just before me.
