How Do I record Gameplay to put it on youtube?

Have you ever wondered how People and company like G4 T.V and IGN make Game trailers and record there own game play? They all use something known as a " Capture Card " which can be purchased from places such as Bestbuy, Amazon, and almost all other electronic stores sell them.
A Capture card I recommend to use from personal experience is the Pinnacle brand.

I recommend these capture cards for use with: Cameras, PS2, PS3, Wii, Xbox, Xbox 360, and any other device that uses an A/V Cable.

However, To record PC Gameplay for PC games. I recommend using a program called FRAPS.
I use FRAPS myself to record PC Games and it records with HD quility. You can't get a better recording software right now to record PC games than this software. I paid for the full version and it's worth every penny even tho they also offer a free tryout version. It Not only records your games but it also takes Snapshots with the same HD quility.