The easiest way for us to introduce the major scale to you is by discussing the C Major Scale on the piano.
It's called the "C" Major Scale because it starts on C (the root note is on C).
A song or piece that is made up of notes from the C Major Scale is said to "be in the key of C Major". Interestingly, it doesn't have to start on the root note (C in this case), it just often sounds the best if it does.
The major scale is made up of a set pattern. Using the piano as an example (see diagram below), the C Major scale follows this repeating pattern:
1st Note: C
2nd Note: up two keys
3rd Note: up two keys
4th Note: up one key
5th Note: up two keys
6th Note: up two keys
7h Note: up two keys
8th note: up one key (back to C - the scale has now travelled one octave)
On the piano the C Major is the simplest major scale, because it uses all of the white keys, but none of the black ones.
The major scale always follows this pattern: 2-2-1-2-2-2-1.
In Music Theory, a 2-note step is called a Tone, and a 1-note step is called a Semitone