You create a small tile in photoshop or psp or whatever graphic editing program you use. Just a single tile, maybe about 4x4, with a single line on it.
There's tutorials out there for it. Try these:
http://pootato.org/photoshop/patterns_create.phphttp://pootato.org/photoshop/patterns_diagonal_lines.phpWhen you finish the tile, save it as a .gif then define it as your background in your css, as i stated earlier. It will tile across your page creating a background pattern.