A very useful video on How to Tune you Violin or Viola by D’Addario.
Key points are:
Tune quietly – if you over-bow or over pluck it may distort the sound and be incorrect
Tune slowly – the cause of most breakages, especially on new instrument is over tensioning the string. If you tune slowly until you are just above pitch and then bring it back you will avoid snapping the string.
Tune frequently – practice makes perfect and makes playing more enjoyable!