If you only ever learn one scale, then it has to be the major ... C root note, then E, G and finally the next C note. Going up and down through the notes gives you: C E G C G E C. Repeat the pattern ...

The C major scale is made up of all the white notes on a piano keyboard, but other major scales (starting on different notes) include black notes - G major, for example, contains one sharp note, F#, ...