HD DVD is an obsolete format: Toshiba stopped supporting it in February 2008. You still see the players around in discount stores, but no-one is making the discs anymore.
The remaining format for high definition DVD is blu ray.
To play a HD DVD disc (assuming you have one) you need a HD DVD player (it will also play standard DVDs). For a blu ray disc you need a blu ray player (it will also play standard DVDs). A standard DVD player will not play either HD DVD or blu ray, and cannot be altered so that it will play either (a standard DVD player uses the wrong kind of scanning laser).