As you can imagine, tuning is extremely important with the guitar and any other instrument. Every guitarist needs to know how to do it, and how to do it effectively and quickly.
There are different ways you can tune your guitar, using many references. You can use an electric tuner, a piano, or other instruments. Most importantly though, you need to be able to tune just using your ear.
Before you learn exactly how to tune, you need to make sure you understand a couple things...
1. The word "pitch" refers to how high or low a note is. In other words, it's simply the relative state of the note.
2. In regards to tuning, the word "sharp" means the note is too high.
3. Also in regards to tuning, the word "flat" means the note is too low.
4. When a string is sharp, you need to loosen the tuner knob to lower the pitch. On the other hand, when a string is flat, you need to tighten the tuner knob to raise the pitch.