Whoo, this is a fun subject.

For metal a lot of people like tuning their guitar down, but I don't think it's necessary.

Do you like marilyn manson? Some of those songs should be pretty easy for you to learn and mess around with.

Don't bother with open chords as much as with other techniques, the amount of gain usually used with metal makes open chords completely worthless. They're good to learn at first because they build flexibility and strength, but keep in mind they're not what you'll be using for metal.

The notes you play in metal quite often aren't as important as the rhythm and tone/distortion as long as they're in key.

It's a good idea matching the vocalist in metal as best as you can. Disturbed does a very remarkable job of this, it's one of the reasons I love them so much.

Learn those scales. They're boring, but they're definitely worth it for the theory and familiarity with the fretboard they give you. If your attention span isn't obsessive as some guitarists try working on the harmonic minor scale, it's definitely a fun one.

Don't practice with a whole lot of distortion. It hides bad habits you might be developing.

Try to develop your ear so you can match pitches decently. With metal a lot of the guitar work is as a rhythm instrument using not a huge variety of chords(usually), so if you can learn what fits with what that's most of what you'll need to be really good. It's hard developing an ear for pitch, but it's incredibly useful. Learning to tune by ear helps this.

Don't concentrate so much on the chromatic scale. It's not as useful or as fun to work with as most other scales, like the harmonic minor I mentioned earlier.

Instead of learning yankee doodle like out of a book, what you might try is learning it by ear. Try listening to someone singing sometime and then teaching yourself to play the vocal part and its nuances by ear. If you can do this it's going to be a huge advantage and tool for you. It's hard, yes, but definitely worth it.

Hopefully some of that helps. Welcome to the forum :)