Watch the Tuning the Guitar online guitar lesson by Bruce Arnold from Jazz Guitar for Beginners
You should tune your guitar every single time you pick it up. Guitars (particularly cheaper ones) tend to go out of tune quickly.
Make sure your guitar is in tune when you begin to play it, and check the tuning frequently while you're practicing, as the act of playing the guitar can cause it to go out of tune.
At first, it may take you five minutes or more to get your guitar in tune, but the more familiar you are with tuning, the more quickly you'll be able to do it. Many guitarists can get their instrument roughly in tune in about 30 seconds.