The debate between using a shoulder rest or not can be a passionate one. I recommend the use of a shoulder rest for all levels of players. Those who do and do not use one can both agree that the instrument does not sit on the shoulder, but instead rests on the collar bone. Most new students will start with a sponge rubber-banded to the back of the instrument. No matter what kind of rest you are using: a sponge, shoulder rest, cloth, or nothing at all the violin/viola should touch the collar bone. If the player has a long neck, a taller chin rest would be much more beneficial than a taller shoulder rest. If you use a shoulder rest, remember that it's only purpose is to support the space between the shoulder and the instrument, and not to prop it up.
The term "chin rest" is the cause of most of the confusion among new students about this violin/viola accessory. The "chin" should never actually touch the chin rest. The side of the jaw is what rests on it, and it and helps support the instrument. Putting the chin on the chin rest will either cause a tremendous amount of neck pain, or will draw the violin/viola down towards the front of the body.
Every beginning student, including myself, has a habit of bending their wrist as if holding up a pizza tray. Most students do it because they feel as if it is the only way of supporting their instrument. A good shoulder rest will help prevent this, but the student must also realize that the violin is supported lightly by the thumb, and the third knuckle of the index finger. I say "supported lightly" because the violin should never be gripped. There should be a straight line from the second knuckle of the left hand down to the elbow.
All four fingers should be nice and curved. The pinky will most likely need to build up some muscle to be adequately arched. The shape of the index finger is often referred to as "three sides of a square." Even when playing a lowered first finger (like B-flat) the finger should still look like a square.
There are countless creative ways to introduce bow holds. Many teachers will have students make a "bunny" shape by bringing the middle and ring fingers down to the thumb while keeping the pink and index fingers up. This is a great way of showing that the middle fingers and thumbs should be close together, but forces the student to use muscles that are not necessary. I like to introduce the "flop" for demonstrating bow holds. I ask that the student just flop their hand over the grip of the bow around the second knuckle. I sometimes refer to this as the "zombie-flop" depending on the student. When the student just flops their fingers over the bow the muscles stay relaxed.