Plasmachick, I'll try to give you a slightly better reply than the other guys
There's a TON of information on these forums, but your head will spin as you read it all.
Do you have HD cable? If you have cable or satellite, the first thing you'll want to do is make sure you have a cable or satellite box that can handle HD.
If you don't want to do any upgrading of your components, then you should get either Component or HDMI cables for your DVD player.
None of this is altogether straightforward and truth be told, it's a lot more like setting up a computer. I recently had to walk my friend via email setting up her LCD TV and getting her going with high def and making sure her DVD player was outputing at the maximum resolution.
If you're ready to upgrade your components, pick up a Blu-Ray player (Panasonic makes some great ones, or just pick up a PlayStation 3).
That way you can ditch your DVD player. I dunno about your VHS
. It will work fine on your new plasma, but probably not look so hot.