Chord are named based on the scale degree they're built on. An upper case Roman numeral means the chord is major. A lowercase roman numeral means it is minor. Roman numerals with an 'o' superscript are diminished.

We'll just be focusing on I, IV, and V for now. These are the keys you need to know:

Example: "Mary Had a Little Lamb"

Notes that are not included in the chord are placed in parentheses. These are embellishing tones because they embellish the main chord tones. Embellishing tones usually connect chord tones by step.

Example: "Imagine," by the Beatles

You can add a 7th to any of these chords. It is most common to add it to V, but you'll often also see I7 chords in rock and especially jazz. This peice is in C major. It uses a I7.