When holding the neck of your violin, ensure your wrist is curved and your posture is perfect in order to create a rich tone.
Make sure your left wrist does not "collapse". You should support your violin with your neck and shoulder. Your left hand must be free to move, though you can bring your thumb underneath a bit to slightly counterbalance your fingers.
Keep your fingernails trimmed short. This is important for a clear sound. You should depress the strings with the tips of your fingers on your left hand and you won't do that well with long fingernails. Also, a good vibrato will not be achieved with long fingernails.
Hold the violin in a relaxed manner. The use of a shoulder rest will help. The shoulder rest prevents excessive tension in the shoulder.
Keep your violin up high on your left shoulder with the scroll out to your left rather than directly out in front of you as this can make proper left hand positioning very difficult.
Ensure that you look straight ahead, not down your strings as you play. This will have the effect that it's more the left side of your jawbone that rests on your "chinrest" than your chin.
Your bow should move perpendicular to the strings. Lead your bow with your right wrist. The stick of the bow should be slightly tilted toward the fingerboard (away from your face).
Be very careful to keep your pinky from "floating." Instead, make sure it's firmly on the nut or shaft of the bow but without being "buckled" or bent backwards.
Your bow should move between the fingerboard and the bridge. When closer to the bridge, it will be louder. Play closer to the fingerboard to play more quietly. (For the beginner: the fingerboard is the long, black piece running down the center of the violin.)
When playing quiet sections of music, keep using all the bow but press down lighter.
Practice! Practice will help you progress because everything needs practice. 30 minutes is expected for young beginners. As a guide, college majors are recommended to practice roughly 2 to 4 hours per day. At least an hour a day should be expected as a bare minimum for serious practitioners.