For each key there are 3 chords that are played more than others known as "primary chords". The I - IV - V chords are built from the 1st, 4th and 5th note of a scale.
Many songs, especially folk songs, use the I - IV - V chord pattern. An example is "Home on the Range" played in the key of F.
These chords are built from the 2nd, 3rd and 6th notes of a scale and are all minor chords.
You can play around with various chord patterns to see what other melodies you can come up with.
Modes are used in many types of music; from sacred music to jazz to rock. Composers use it to add "flavor" to their compositions in order to avoid predictability.