Raxel said:
If there's one essential piece of advice that you should know, it's to use ALL your fingers when playing.
I too can't stress enough how important this is. Using your pinky feels VERY weird at first but it is SO ABSOLUTELY ESSENTIAL to becoming good. Even if you never pulll out the pinky unless you absolutely need it, it is absolutely necssary to be able to make use of it.
Other things to think about.
1. Always challenge yourself. It's easy to become content playing nirvana and green day and your friends will probably think you're hot shit for it(depending on how old you are), but you should ALWAYS have something (prefereably alot of things) that you're 'working on' and can't quite do yet. This keeps the instrument interesting, and keeps YOU improving.
2. If something just seems impossible, maybe there are other ways to play it. I find alot of trascriptions to be sonically accurate, but the fingering and layout of an arrangement to be just way off. Alot of the times I'll work out more logical ways to play things and that helps.
3. If you just can't get something, put it aside for awhile. If you've looked at the fingerings and some riff is just a bit too difficult for you to play at your current skill level, put it aside for a month or so and practice other things. When you come back to it, you may find that you can nail it flawlessly.
4. Break things up. Say you're playing some long passage and can't, for the life of you, play it straight through. Break it up and practice it in sections. It may seem logical, but you'd be surprised how many people DON"T do this. And I don't mean break it up into a few measures at a time. I mean break it up into groups of 3 or 4 notes. One thing I find beneficial is to sortof stagger the groupings. Say a passage is 16 notes long. First I'll split it into fourths. I'll practice each beat seperately(say it's 16th notes), so 4 notes at a time.
I'll seperately play notes 1-4, 5-8, 9-12, and 13-16. Then, I'll start grouping them together. I'll practice 1-8 for awhile, then instead of practicing 9-16, i'll practice 4-12, then 9-16, etc. Then lastly i'll practice 1-12, and 5-16. THEN i'll peice it all together. I find this to be very beneficial as it helps to cover any sort of transitions that might take place within the peice.
5. Lastly, realise that there is no song out there that is beyond your reach. 4 years ago I told myself. "Holy shit...John Petrucci is incrdible, he picks 32nd notes....I'll never be able to do that". Well, here I am, and while i'm still no JP, I can play a large portion of his stuff fairly easily. Point being...things that seem impossible to you now, are just that. Impossible to you NOW. With diligent practice and a good attitude though, and the realization that you have the physical ability to outplay anybody out there, in a few years you'll be playing things you wouldn't even dream about now.
edit - and alot of this stuff won't apply to you for now. But as you start to get better you'll see how these tips can help you out and you'll figure out your own little tricks for getting better as well. Becoming a proficient guitarist is as much abuot learning to learn, as it is about learning to play.