When I studied music theory in school my music teacher would consistently drill us on chord construction and quiz us on how to put the pieces of chords together. I have found that this skill is highly useful when applied to the guitar.Knowing how to build chords and knowing the composition of each chord helps guitarists to create chords from scratch.A Chord is a combination of 3 or more notes played at the same time. How to build chords is an essential skill for any type of musician.
Major Triads:

Stacking 3 notes on top of each other creates  a triad. This is the foundation of all chords. We will take a look at the C Major chord and dissect it. This will enable us to see what 3 notes make up this chord.

Key of C Major:

C D E F G A B

We will now select the C note. After you have located the root note you will then select every other note. The 3 notes that build a C major chord are:

C E G

Major chords consists of a root,major 3rd and a perfect 5th.
Minor Triads:

Now that you understand the basics of creating a major chord, we will learn how to create a minor triad using an A minor chord. For the sake of simplicity we will use the key of C major again.

Key of C Major:

C D E F G A B

Select the root note. Which is an A. Then you select every other note until you have 3 notes total. Your A minor chord should look like this:

A C E

A minor chord consists of a root, minor third and a perfect fifth. So basically the only thing you have to do to make a major chord a minor chord is lower the third! If you wanted to make this A minor chord an A Major chord we would raise the 3rd a half step. (C to C#)

Practice building all of the chords in the key of C Major. Then, move on to each key and understand how to build the chords for each key. It is beneficial to understand the notes on the piano, this will help you organize the notes.