Em means E minor lowering the 3rd 1/2 step. Hammer on means to push forcibly down on a string at the proper fret. This is usually done with the left hand but in shredding it can and usually is done with both hands.

I don't know how much you know about music so don't atke offense at the next explanation:

Each Fret represents 1/2 tone in pitch so it would be C (C# Db) D (D# Eb) E F (F# Gb) G (G# Ab) A (A# Bb) B. Notice there is no half pitch B & C and E &F.... if the music is going in an upward direction you would use the sharps # when moving down you would be using the flat or b.

Playing on your guitar Standard tuning and starting from the bottom or the thinnest string to the top is E B G D A E. 1 hole not consists of 2 - half notes. Sounds dumb except when you don't have the half notes between B & C and E and F. So if you would take the 1st 3rd and 5th of these notes you would make a Major chord. you can add the 7th note and make aMajor 7th Chord and if you drop the 3rd 1/2 step (one fret) you have a minor chord

The first thing you should do is map out all of the notes on the fretboard and start to mak your own chords. Also one of the nicest parts is using a barre chord because if you can play it all you have to do is move it to the position of the 6th or 1st (they're the same) starting with the 1st note of the chord.

When you get to the 12th fret all the chords start over again, just an octave(going form one note to the same note like E to E) higher.

Learn EMaj chord Em chord and EMaj 7th chord and also AMaj, Am, and AMaj 7th. and learn to barre these chords and you'll be playing songs in no time.

Hopes this helps some, there are probably books in the library to teach these lessons to you. Also pick up a guitar magazine with songs and there will be a page describing all the ways to play a string and how it is done. Hammer- on hammer-off, bend 1/2, bend whole and etc.

Thanks,
hollis_gov