What code can you use to make text move across the screen?

I dont think HTML can do that. you would need javascript to do that, but there is one marquee tag in html that can do this.
http://www.mountaindragon.com/html/marquee.htm

Please note, you are better off with javascript, as marquee tags behave differently across different browsers.
Search on google and you can find many javascript codes which you can use.
Something like this, http://www.google.ca/search?hs=Nl1&hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&q=vertical+text+scroll+with+javascri pt&btnG=Search&meta=

Yeah, some browsers don't display marquee tags properly

<marquee> then text </marquee>

<marquee>xyz</marquee> works in IE, but not in FF

Yes, in IE it scrolls as expected, but in Netscape it blinks not scrolls (Atleast i think so, but that was 2 years before i tried that, not sure if its still that way).

yes html uses marquee
<marquee behavior=right>Right Moving Text</marquee>
<marquee behavior=right speed=50>Fast Right Moving Text</marquee>

Use javascript to move text otherwise by using
document.writeln("line of text");
document.reload();
document.writeln("<p>second instance of text");
//and so on and so forth

Try using MS Frontpage!

Do this:
Insert > Web Component
Select Marquee, Click finish!

And your done!
You should have plenty of options there to get you started!

DHTML used with div tags can do it, but again, you might run into browser issues.