How can I lower the resolution on Skyrim?

I bought Skyrim for PC from Steam. I want to lower the resolution to make the game run more smoothly on my computer, but there is not an option for that. How do I lower the resolution?How can I lower the resolution on Skyrim?
When u launch the skyrim launcher,before clicking on play click options and u can change the resolution or u can change it in the display option in the game.How can I lower the resolution on Skyrim?
Go to this file:

C:\users\ YOUR USER NAME \my documents\my games\skyrim\SkyrimPrefs.ini



There will be an entry (without quotes) "iSize W=XXXX" and "iSize H=YYYY"



Replace the XXXX with your desired screen width (i.e. 1920) and the YYYY with your desired screen height (i.e. 1080)



Also make sure "bFull Screen=1" is present. If it reads "bFull Screen=0" change it to "bFull Screen=1"



Then save the file.