I don't know why certain persons are trying to teach you musical theory at this point.
The guitar is (in my opinion and with some experience) both the easiest and most difficult instrument to learn.
FORGET the scales and theory for now. Learn about half a dozen chords and you will be able to accompany yourself to about 100 popular tunes which have simple harmony.
So it is the easiest instrument to yield tangible results in a short time.
On the other hand, when you get into the fancy stuff, you will find that the guitar was NOT designed with the human hand in mind. You will have to teach your fingers to distort themselves into positions which they were never designed for.
So it's easy to get started on the guitar. Another instrument that is easy to learn is the recorder. So you could play the melody on the recorder while your children played the accompaniment on the guitar.
To play most other instruments, like the keyboards for example, requires regular practice for a number of years. The brain has to be slowly and painstakingly trained to transfer those little black dots and other symbols on the sheet music into appropriate movements of the fingers, WITHOUT any conscious thought involved.
Each instrument (piano, strings, brass, woodwind) has its own distinctive difficulties which have to be mastered. To achieve a satisfactory result with any of these instruments requires regular practise over a number of years. I think generally speaking it is easier for a very young person to pick up anything -- like different languages for example -- than for a mature person.
I would not agree with Maxwell that motivation is the main prerequisite. TIME is the most likely limiting factor for a mature person.
Some people have pointed out the scientific basis of music. Here is a brief item which I wrote some years ago.
I have often heard it said that if a person is good at mathematics, then they will also be musical. There is no random coincidence to this relationship between music and mathematics. My comments are limited to Western music as I have not explored other forms of music. The basic premise of my discussion is that music was not so much composed by brilliant musicians, but evolved inevitably according to fundamental mathematical principles.
The crux of my postulation lies in the fact that, the simpler the ratio between the frequencies (cycles per second) of any two or more notes, the more pleasing the combination to the human ear. I am also assuming that the creation of music preceded both the development of a system to write it down (notation) and an understanding of the theory that developed to explain the composition.
The development of the eight note 12 semitone octave fundamental to Western music was inevitable, as it is based on precise mathematical ratios. The octave represents the simplest of all ratios, 2:1. But as we continue through the common intervals, we can see the principle of ratios demonstrated further. Thus the interval of a fifth has a frequency ratio of 3:2, the fourth 4:3, and the Major third, 5:4.
Further intervals are the Minor sixth (8:5), Minor third (6:5), Major sixth (5:3), Major second (9:8), Minor seventh (16:9), Diminished fifth (7:5), Augmented fourth (10:7), Major seventh (15:8) and Minor second (16:15).