ultimate-guitar.com
howtotuneaguitar.org
Some people really try, but really have a hard time learning guitar. I recommend they get some guitar lessons from a personable and knowledgeble guitar player. My Uncle Phil is a great example of someone you would want to learn guitar from:
http://www.philwestfall.com/Guitar_Instruction.html
How to tune a guitar:
Useful to teach you a lot more about tuning than I can. The valuable tool on this site is the tune-o-matic that you can use to tune to pretty much anything you'll ever need. Use this site to tune in drop D when I explain it in the next section.
Ultimate Guitar:
Has some interesting and useful articles plus thousands of tabs. Tablature tells you exactly what notes to play and is probably the best way to increase your skill in guitar. It is worth mentioning that you can also buy a program called guitar pro and download a special kind of tab from this site for free. Guitar pro is an extremely useful tool for learning guitar as well. This program plays all the notes of a song so you can read the tab while hearing the song. My favorite part of the program is the speed trainer. You can tell any part of the song to play at a certain speed, and you can have it repeat each time getting faster in increments. Playing along to the speed trainer is like playing guitar hero, except without those annoying sounds every time you miss a note. Practicing in this way allows you to learn a song thoroughly and at a rapid pace.
Playing tips and techniques
If you're an advanced player, most of the info on this site you probably already understand. But for beginners, you sometimes miss important simple techniques that hinder your playing. These tips are designed to cut out the time wasted so you can practice without interruption (well at least without self interruption). If you find your guitar skill lacking, come back to these tips and focus on each one to see if you can improve.
1. Two hands become one
Play as fast as you want with one hand, but if the other can't match it, it's going to sound like shit. As you improve, try to notice if your picking or fretting is slower and improve the slower. Otherwise you won't get much better at the other.
2. Picking
The pick should be held between the thumb and index finger. Your hand should be as comfortable as possible while keeping the point of the pick straight on to the string at a 90 degree angle from the guitar. Accuracy will help you increase speed by reducing the distance between each picking motion. Less distance = faster picks. Use a metranome and pick as evenly as you can to improve.
3. Fretting
Fast fretting will come to you with practice. It is important to press firmly in the middle of the fret and not on the bar in between frets. Do not press too hard as you will distort the sound. Playing solos or songs with a lot of notes will increase your fretting ability. You will notice sometimes after you learn a song that you are playing the right notes but your fingers were in the wrong position. Recognize this and learn from it because it is the art of fretting. You'll play faster when your fingers aren't all tangled up. Try to move up and down the neck in patterns, keeping in time with your picking hand while using a metranome.
4. Tablature and Practice!
You won't get better at guitar by just listening to music all day. Try to devote some time to playing every day until you feel like your guitar is your third arm. Looking up tablature is a great way to get practice in and learn new songs. You can find tabs online or you can buy books of them at music stores. When learning a new song, play as much as you can the first day but don't worry about a part if it's too hard. I find that when I come back to a song the second or even third day, the hard parts become easy. If it's still too hard than move on to another song and go back to it later. Do more than just learn songs. Take notes and progressions that you can play well or like to play and try to make your own song or move them up and down octaves. Mix it up and experiment. Feel confident in your ability to play.
5. Tuning and then drop tuning
Make sure your strings are in tune to get the best sound possible. The fifth fret of every string is tuned to sound like the next string played open. Playing a string open means playing it while the other hand presses on nothing. In tablature, it is marked as the number 0. In standard, the notes of the strings from the lowest string to the highest string are EADGBe. Don't worry about all these letters yet, just try to memorize them for now. Now for the fun part; tuning in drop. I said before that every fifth fret is tuned to sound like the open note of the next higher string. The main execptions to this are: 1. The fourth string on the fourth fret sounds like the next open string 2. When you tune in drop the first string on the seventh fret sounds like the next open string. Each fret is a half step, and two frets is one whole step. In drop tuning, the lowest string is tuned one step lower than the rest, making the 7th fret sound like the next open string. Where does that bring us if we're trying to tune in Drop D? Well, from standard tuning, only the lowest string is tuned down one step. Why tune one string lower you ask? Drop tuning changes the way you play chords on the first two strings. A conventional chord in standard tuning would look like:

Now with drop tuning, you can play that same chord formation anywhere on the first two strings like this:
Since the tuning is lowered on the first string by one step, the note on the first string is one step from where it previously was. These new chord formations on the first two strings allow you to play the same chords faster by barring one finger. So there it is, regular and drop tuning.
(These chord charts courtesy of my Uncle Phil.)
6. Strange Noizes
Playing metal, as well as many other types of music, requires you to know how to squeeze all the sounds you can out of that guitar. There are plenty of ways to push extra sounds out without bashing your guitar against the ground while it's on.
Palm mute- A metal players best friend. Palm muting is achieved by resting the edge of your palm from the pinky to the wrist on the strings while playing. Now you know why you love metal so much... The palm mute is more sustained the closer to the end of the strings your palm is.
Bend- Move the string while playing a note to give it a wavy sound. Pull it in different ways to change the sound of the bend. Also you can use a wangy bar with many guitars but it can make your guitar out of tune.
Pinch harmonic- This is how we get notes to scream. This technique is tricky to learn at first and also hard to explain. But when pulled off it gives a wicked sound. You turn the pick sideways when playing a note and let the string bounce off the pick and then your thumb quickly. Your thumb has to catch the harmonic just right which is a sweet spot along the string. Try bending the note while doing a pinch harmonic for some awesome sounds.
Well, that's all the tips I have for now. Hope they helped.
Shred667