How to create melody with scale?

Im not sure if iknow what im talking about but a scale has to start on the tonic or root note or else it goes out of key. I cant find anything that measures a scale starting at the 2,3,4,5,6,7 and the octave going out of the scale so have to keep returning to the tonic noot without changing the key. AM i missing somthing if this makes sense