Understand the ways scales are notated. A scale can be written on the music staff or written in letter form: C, D, E, F, G, A, B, C. Scales also can be written by the distance between them, in whole or half steps. The is the step structure for all major scales is whole step, whole step, half step, whole step, whole step, whole step, half step. You could start on any note and follow this structure to find the notes in any key.
Locate the sixth note of any major scale and follow the notes in ascending order. This gives you the relative minor key. Because the sixth note of the C major scale is an A, starting from that note and reading in ascending order would give you: A, B, C, D, E, F, G, A.
Practice playing your scales. The best way to become familiar with reading them is to play them. You can purchase a scale book in a music store and keep it handy as you learn to read scales. If it helps, sing the melody most of us are familiar with as you practice: do, re, mi, fa, so, la, ti, do. Keeping this melody in your head as you read scales may help you visualize them better.