the main rule is to use whichever fingers feel most comfortable, but in general it is also best to try to minimise movement of your hand, so instead of using a 1 fingered approach, it is best to try to get all your fingers involved. Some simple tunes allow you to use finger 1 on note 1, finger 2 on note 2 etc.. but for many songs, using other fingers fit better with the rules about feeling comfortable and minimising hand movement.
Again when playing notes 6 and 7, it depends what notes come before them as to where your hand position will be starting from. If it doubt, simply use whichever fingers feel most comfortable.
For example, the melody in Amazing Grace starts 5 1 3 1 3 2 1 6 5 – I would use my fingers as follows:-
1 second finger
2 third finger
3 fourth finger
6 second finger (you need to move it from the previous position)
with regards to playing chords in the left hand, ideally the little finger, middle finger and thumb should be used, however some people may find different fingers more comfortable. It doesn’t matter which fingers you use, as long as your hand feels comfortable.