The 1080p resolution is only available on Pay per View and your TV must be a newer model capable of accepting the 1080p/24 signal. If you go to setup under resolutions and try to check the 1080p, the system will check your TV to see if it's compatible. Don't try to force the resolution. If it's accepted, you can try a 1080p program without cost by going to the PPV portion of the guide and finding a movie that has just started. When the popup asks if you wish to purchase, ignore it, the program will play for a few minutes without actually buying it.
Note that on TV's that won't accept the 1080p, you'll get equal picture quality from the 1080i so it's of no concern.