Learn to play power chords. Power chords are a staple of hardcore metal because they are easy to form, they can be moved up and down the neck without changing fingering, and they don't contain the third note of the scale, leaving room for lead guitarists to play major or minor guitar solos. Place your index finger on the third fret of the thickest string of your guitar. Place your middle and ring fingers on the fifth fret of the next two strings below the thickest string. Turn your amplifier up and play these three strings together. This is a basic G power chord.
Practice playing the G power chord, moving it up and down the neck of your guitar without changing the way you're holding your fingers. The name of the chord is the same name as the fret your first finger is on. If you keep the same shape with your fingers and slide your hand up so your first finger is on the fifth fret and your index and ring fingers are on the seventh fret, you will have an A power chord.
Practice metal techniques while you play the power chords. One technique is to play a chord and let it ring out for several beats. The louder the volume on your amplifier, the longer the chords will ring. Another technique is palm muting. Play the power chords with a steady down stroke of the pick, but rest the side of your hand lightly against the strings to prevent them from ringing. This technique is often used for rhythm in hardcore metal songs.
Play single note licks (a series of notes strung together) and work on metal lead guitar techniques such as playing a note and pushing the string up or down to raise the pitch (string bending). Practice tapping solos. To do this, play the single notes on your guitar with the fingers of your fret hand while you play notes higher on the neck of the guitar with your picking hand. At high volume, you can tap a string with the index finger of your picking hand and push it down to play the note. This is called tapping and can allow you to play more notes in a wider range.
Combine the lead techniques with power chords and chord-playing techniques to acquaint yourself with both rhythm and lead playing. Practice by starting slowly, then gradually increase the speed you play. Much of the music associated with hardcore metal is played quickly. The faster you play these basic techniques, and the louder the volume, the more metal they become.