First, decide how much money you want to spend on a violin. If you are buying an instrument for a child, take their level of commitment into consideration. If they like to play the violin but do not seem that committed, purchase a less expensive instrument. On the other hand, if your child is very enthusiastic and plays the violin frequently, than spending more money on a violin would be appropriate. Next, contact any music stores and instrument dealers in your area.
Dealers generally carry higher end instruments and music stores carry lower end ones.
Ask them if they carry instruments in your price range and set a date to view them. Most stores and dealers will have several violins that are the proper size and in the right price range. Calling in advance gives them a chance to prepare the instruments to be played. A music instructor or a store employee should help you determine what instrument size you will need. You should try a handful of instruments so you can compare tone quality, sound, and volume.