Finger movement
“When you work at a microscopic level, you have to control every part of your body movement: your fingertips, your joints, the pulse in your fingers.” – Willard Wigan
Enough theory for now! Let’s pick up the guitar and see how we can start playing it.
Before we start, though, I want you to know one thing that would have been great for me to know when I was first starting to learn.
Guitar techniques, as they are usually taught, are not the definite way in which you have to play. I recommend you learn the basic techniques first in order to get a base for your specific learning, as you will certainly discover afterwards that some changes will be easier for you to play. This is fine.
So now, let’s start.
By now, you should already know how to hold your guitar and how to place your hands on the neck. If you missed that lesson, you can find it here.
You should also know that it does not matter whether you are right or left handed. In any case, just flip everything over to the other hand.
Fret hand
The first thing you should know about finger movement is that in general when playing guitar you always want to make your movements as efficient as possible, that means that you have to make the shortest and minimal movements possible. I’ll show you some examples on this later on.
So, of course, in order to play the instrument we will have to hit the strings while pressing on different frets. When you hold on a fret, you will almost always want to do so with the tip of the finger. There are only a few exceptions to this rule, which we’ll see later on as we progress.
Not just using the tip of each finger, but also doing so in this way:

Notice how the last phalange of the pressing finger is straight into the fretboard. This is how you should press with your fingers on most techniques.
Please, do not do it like this:

It might look similar, but believe me, using this technique will strain your fingers a lot and is a risk for injury sooner than later. Notice how the last phalange is bent outwards. Please, don’t do it!
Let’s start with our standard hand and finger position:

When moving fingers from a fret to another, you want to lift the finger from the string as little as possible. This means that the movement needed is less and also means that it will be faster. It’s actually quite difficult to control this, so don’t worry too much about it at this stage, but do not forget is, as you will have to train yourself to do this better as you continue learning.
As a general rule, you’ll want to keep your knuckle line parallel to the fretboard. Avoid doing the following:

The reason this is not a good idea is that it removes part of your reference to the whole fretboard, and also involves additional movement that can slow down your playing. Some people like to do this sometimes, especially when climbing up the fretboard quickly, but most of the times you will want to keep an eye on this and avoid it.
Now, take a look at the following photo:

Notice anything weird? I hope you do, because this is also something you want to avoid from day 1. Check out how the fingers are all curled up and much more tense than with our standard position. This position will probably not hurt you, but it will hurt your playing quite a bit, since your fingers are much more restrained and have less leverage on the strings.
On the contrary, focus on moving all your fingers seamlessly like this:

OK, so by now we’ve already know how to use our fret hand. Coming up, let’s see what to do with the picking hand so we can start picking the strings.
Picking hand
When it comes to the picking hand, there are two main ways to use it:
Fingerpicking
This is a technique in which you will use your fingers to pluck a string, or group of strings. The most common way to do it is by using your fingerprint to catch a string, then plucking it and letting it ring, like this:

You can also use your fingernail to push and pluck a string, although that tends to be more difficult, so I recommend you don't worry about it right now.
Of course, the thumb is there to be used too! You can use your thumb in the same way as you would use the rest of the fingers, although in order to pluck the string you will need to push in the opposite direction as with the other fingers (duh!).

Usually you will want to use your thumb to pluck the thicker strings (6th, 5th, and 4th) and the rest of your fingers to pluck the rest. We'll see why this works best later.
So, how do we place the hand and wrist in order to fingerpick? Take a look at the following picture:

You have to bend your wrist in order to get your fingers in a position where they can pluck up or down. If your wrist were to be straight, the movement of your fingers would be at an angle to the strings, and it wouldn't work well.
A word of caution, however, you should not bend the wrist too much, as doing so will result in pain sooner than later and you might be injured. Listen to your body.
Using a pick/plectrum
The pick is great for very fast picking, but it also has the drawback of being a single element you can use, in comparison with your five fingers, so there's that.
Picks come in many sizes, materials, shapes, colors, fire or water type (ha!), etc. We'll see how to use picks in more depth later, but for now let's take a look at how to start using a pick.
Grab it with your thumb and index finger. Make contact with the string and push it so it rings. For now, focus on using the flat side directly in contact with the string, like in the following photo:

The plucking motion can push the string downwards or upwards, but for now focus on using it downwards as in the photo, as it will be easier to get the hang of it.
Placing the hand and wrist is different than with fingerpicking:

Because while picking with the pick you can control the angle of the plectrum with your fingers, keeping the wrist straight will give you much better results, and will also be much more comfortable.
Congratulations! You now know how to handle the guitar appropriately.