Learning guitar these days pretty much always involves reading tab.  If you are just starting and don't know what tab is, it's simple:  It's a 6 lines, and each line represents a guitar string.  The numbers written on the line indicates playing that fret, on that string.  The bottom line is the low E string, the top line is the high E string.  Here's an A minor bar chord:
----5----
----5----
----5----
----7----
----7----
----5----

The internet offers a huge repository of tabs, for pretty much any song you can imagine.  In my teaching, I provide tabs of whatever we are studying.

There is something to be said for traditional music notation.  It is a denser, more information-rich means of conveying musical data.  Each note can display its pitch, its duration, and its relation to the notes around it-- it is very difficult to perceive intervals from guitar tab, but it can become second nature to perceive intervals in traditional notation. 

Sght reading music is itself a discipline, and I only teach it when the student feels the need.  Of course, I encourage it.  But, tab is more expedient in terms of showing guitar riffs and licks, and you use your ear to determine the rhythmic intricacies. 

A great program to get is Guitar Pro.  It is the standard for getting tab on the computer.
 
There are many websites that offer lot's of tabs.  Here's a nice one:  G Tabs
The Guitar Pro website has a page listing many more of these tab sites.

Here's a pretty sweet shred lesson from a guitarist from Children of Bodom.  He points out some real, standard shred techniques.  Try to critically observe his fingers-- this isn't the best example of effortless playing.  But he gets the job done, and if you like his band, then you will be especially excited.

In music, the distance between any 2 notes is called an 'interval'.  There is a helpful way to memorize intervals, and here's an example.  If we are in the key of C, then C is 1,
C = 1, D = 2, E = 3, F = 4, G = 5, A = 6, B = 7, C = 8.
The interval between C and A is a 6th.
Now, let's invert-- what is the interval between A and C?
A = 1, B = 2, C = 3, D = 4, E = 5, F = 6, G = 7, A = 8
The interval between A and C is a 3rd.

The interesting trick here, is that these 2 intervals (C to A, A to C) are inverse of each other, and the sum of the intervals is 9. 
6 + 3 = 9

This works for all these intervals.  Here's another example:  What is the interval between C and F?
a 4th
What is the interval between F and C? 
Well, 4 + x = 9,
9 - 4 = 5
The interval between F and C is a 5th. 

I'll let you figure out the rest of the intervals-- in all keys, too!  Refer to the circle of Fifths to learn how to spell all the scales you need.

I had a profound realization recently, when I was studying chess moves.  (This should indicate the magnitude of my nerdiness, if nothing else does.)
In chess, the board looks like this:

Notice, going from the White king to the Black king, there are 8 rows (or ranks).  Just like in music--  The first row is 1, and the 8th row is the octave.  Like in music, when you inverse an octave, it's just like a mirror image-- high C to low C.  In chess, when you inverse the octave, and you make the Black king your own, and the 8th row becomes your back rank.

Look at Whites 2nd rank.  All his pawns are there.  Look at Blacks pawns-- where are they?  All on the 7th rank.  In chess, the inversion of the 2nd rank is the 7th-- White becomes black, but its weirdly the same. Notice=
2+7=9

Just like in music.  In chess, the center of the board is crucially important, and for black and white both, there is a center line.  When you cross this line, you enter the enemies turf.  For White, that is the 4th row.  For black, its the inverse-- the 5th.
4+5=9

The same principle applies going side to side.  The columns, or 'files' go from a to h-- think of 'h' as the octave of 'a'.  The inverse of file 'a' is 'h'.  The inverse of file 'd' is 'e'.

I love the symmetry here, amongst the numbers, and amongst two wildly different yet venerable pastimes.  Some people say that music is the emotional expression of math.  And others say that chess, despite its rigid rules and logic, is ultimately an artistic expression.  It's almost as if they are inverse of each other.

Great article here, describing how to discern a good teacher from not so good.  Happily, I have been using similar terms to describe my own aim in teaching.  As the author says, the goal is to empower the student to achieve their own musical development.  I really like how he pegged that essential quality of good teachers.

In my own life, I have been fortunate enough to study with some great teachers, but I also appreciate those who have been not-so-great.  Those bitter experiences I have had are just as valuable for my own teaching, and have taught me the importance of listening and trying to help, without involving my own ego.

Anyways, it's a nice article.  If you are seeking an instructor, give this a quick read and then ponder the instructors available to you.

"The True Teacher" by Jamie Andreas
www.guitarnoise.com/lesson/the-true-teacher/

Found a great little online trainer for locating notes on the fretboard.  Practice a little with something like this, every so often, and you will see your skill increase.  Then, send your thanks to the programmer is, and try it on your instrument.

Fretboard Warrior

If you are an iphone or ipod touch owner, then there are a few useful apps available for musical training.  I'll list a couple I use regularly:

Karajan Ear Trainer
This is far and away the most bestest, most usefullest iphone apps I have found for any purpose.  It is also one of the most pricey, at $15, but I find it to be worth the money.  It plays not only simple intervals, but also chords (triads and 7th and more), scales, single pitches and even BPM.  It has a a few different sounds, a nice layout that has an actual keyboard in landscape mode, and a report on your progress.  You can customize the program in many ways, very easily.  Highly recommended for both beginners and advanced players.
www.karajan-eartrainer.com/en/

iReadMusic
This app is a really solid tool to help learn to sight read traditional notation on the guitar neck.  It has several modes, I use  'Play that Note!'  In this mode, notes on a treble (or bass) clef are displayed on top, and the fretboard is displayed on bottom.  The frets are a bit tiny, but it's actually precise enough to go through entire scales.  The program has a lot of customization available, including other instruments (piano, bass and banjo), sounds, and types of drills.  It's only $2, so I think it's worth the investment.
www.glpsoftware.com/

Metronomes and Tuners
There are dozens of metronomes and tuners out there.  I found two for free (my usual pricepoint) that work well.
iTick
Guitar Tuner
Tuna Pitch is a $2 tuner that seems to do the job.

GigBabyFree!
How is it that my wallet-sized phone can house an entire four-track recording studio?  People over the age of 30 may understand my wonder.  In any case, here is a free recording app that has 4 tracks and some bells and whistles to boot.  Really fun, and others like this get to be more expensive.
www.iometics.net/GigBaby!/

This lecture transcript has been floating around now for a while, I thank a friend for forwarding it, one of those 'inspiring email forwards', but for musicians, and actually inspring. 

Karl Paulnack is on faculty at the Boston Conservatory, and he gave a commencement speech to the new students of 2004.  He likens the role of a musician to more of a therapist and medical professional than entertainer.  Although he is speaking to future professional musicians, I think the message is also relevant to anyone who struggles to learn to play.  The act of playing your instrument, even alone, is a powerful means of experiencing music.  all the benefits of music can come through even the most basic of practice regimens (if you make sure to keep you practicing fun!)

Here's a link to the article where I read it:
http://greenroom.fromthetop.org/2009/03/11/karl-paulnack-to-the-boston-conservatory-freshman-class/