In a stave system
Two notes on a neighbouring line and space form a second. Depending on where the second is located, it can be a minor second (m2) or major second (M2).
On a keyboard
If there are no other keys between the two marked ones it is a minor second. If there is one other key (black or white) between the two notes it is a major second.
Examples
Beethoven's For Elise
Happy Birthday
The Beatles' Yesterday
The second in a C-Major scale
In a C-Major scale all neighbouring notes are seconds. In the image below the minor seconds are connected with two lines.
