Alternate picking is the way you pick single notes on a guitar most of the time. Mainly this is for speed and to set yourself up for the next note.
A lot of the time in this style picking the up beat and stroke of the pick is an eighth note. Using a metronome will tighten up your timing.
A good way to start training your picking hand is to use some slow simple picking, you don't even have to finger any notes.
Just start playing a note with an down up motion in time with the metronome. Play in time and don't worry about speed that will come with accuracy.
Here is a link that will take you to my metronome page where you can find some tips on how to use one.
Here is a link for holding a pick. To some of you this may sound stupid but it can help your playing when you get into more advanced things.
The first way you should learn is to play one string and stop in between the string you played and the one below it.
Practice this until you can control the pick from hitting the string below it.
It may be simple for some of you but not for others. There will be some aspects of guitar playing that will be easy for some and hard for others, this is normal and doesn't mean you can't learn guitar. Not everyone learns at the same rate.
Every thing you do for the down stroke applies to the upstroke but in reverse. The upstroke may feel a little awkward at first but it will become 2nd nature after a bit of practice. Here you want to pay attention to the grip you have on the pick because this is when you will most likely drop it in the beginning.
Concentrate on every little move until you start to do it automatically and check yourself every now and then. It's easy to develop bad form by being lazy that will affect your playing.
If you can watch yourself in a mirror or better video yourself. Video is better because you don't have to watch until you review the video. Just like the Pro Football players do on Monday after the Sunday game to see what they did wrong.
A neat thing happens when you play a string. As you pass through the string it gives energy to your pick movement. This extra juice you will put to use to propel you to other strings and notes.
It's the rebound force of the string although I don't know what it's called in Physics' terms. It may be inertia, an object in motion tends to stay in motion, I think that's what inertia is.
We can redirect this energy to our advantage for alternate picking.
This is why in the beginning stopping in between strings with out hitting the other one may be a challenge for some.
A little practice will get this straightened out. Your just stopping motion but in very short distances.
Practice stopping in between strings on the first measure with down strokes and then practice your alternate picking on the 2nd measure.

Muscle tension will occur when you play guitar. You must pay attention to this tension and relax your muscles until you do it without thinking about it.
Tension will cause mistakes and make your muscles get tired faster. The main thing about muscle tension is being aware of it so you can do something about it.
Your tension can be anywhere it doesn't have to be in your arm, your shoulders or stomach is a good place like getting butterflies before a performance. Check your whole body and relax. Take a few long slow deep breaths.
As you play you create muscle tension just by doing it. It's just like exercising but you're using your arms and fingers.
There will be times in the music where you can relax your muscles. Play a different version of a chord, this changes the muscles you are using.
The more attention you give to this tension now will help your playing later. It's like muscle memory except you are conditioning your mind and body to react to what you trained it to do. Mind Control.
After you can play some eighth notes like in the example above your ready to expand to other strings that aren't next to the one your playing.
This is where alternating come in handy for speed. This is also useful for playing a picking pattern in a chord much like fingerpicking.
The following list should be practiced until you can go from any string to any other string without delay.
If you practice these moves slowly and watch your hand and pick to make sure you don't hit other strings you will have half the battle of playing guitar won.
6 is the top heavy E string
Practice playing these the following ways
Down Stroke - Down Stroke
Down Stroke - Up Stroke - Alternate - Down Start
Up Stroke - Down Stroke - Alternate - Up Start
Up Stroke - Up Stroke
Some won't be practical for regular playing but good for picking practice.
After you get comfortable with these try playing them with different chords you know and combine two or more together to make a pattern.
