Violas are commonly mistaken for nothing more than slightly larger violins. Although both are played horizontally under the chin, the viola produces a deeper and richer sound, and is tuned a 5th lower than the violin. Buying a viola is similar to buying a violin, though, as you'll have to pay attention to sizes and other accessories that share a likeness between eachother. Shopping for your first viola can be a daunting task, but if you're sure that the student you're buying for is commited to learning the instrument, it can also be a rewarding experience.
Parts of the viola
Violas and violins have many (if not all) of the same parts, just in different sizes.
How to find the right viola for you
The most important thing you can do for yourself before you shop for a viola is to set a budget for yourself. Knowing how much you can and can't spend will certainly limit your search, but can also point you in the right direction. Having a price in mind will also be indicative of the fact that you've done your research, know what you need, and how much people should be paying for it.
Because many students have trouble stretching their fingers to reach all points on the fingerboard, fractional violas are extremely popular. Stores and schools alike offer violas in sizes 3/4, 1/2, 1/4, 1/8, and even 1/16 for students with smaller hands. The best way of finding out what size you'll need is to go out to a handful of stores and try some violas in person. This will give you an opportunity to see what's available to you within the budget you've set for yourself.
New vs. Used
Buying new guarantees that you will be the first person to play the viola in question. However, a used viola will sound just as good, and can be a huge money saver. Buying used comes with a few risks, so there are a few things to look for. When looking at a used viola, check for cracks in the body. Small cracks won't alter the playability or sound of the instrument, but a substantial crack will make it impossible to use. Keep an eye out for loose pieces that need to be replaced. Though most pieces can be replaced easily, the price should be reflective of the condition the instrument is in at the time of purchase.
Bringing a teacher or having a used violin looked at by a professional before you buy it is only beneficial to you. You'll have your viola for a long time, so even if you have to pay a little bit to have it checked, it will be worthwhile when all of your child's classmates are sending their broken violas in for major repairs. Being that it may be your first time buying a violin, sales people and/or independent sellers may try to overwhelm and take advantage of you. Having someone with more experience on hand will prevent this from happening.