You can ask the shop for a recommendation if you aren't sure. These are generally only intended for younger children, so be sure the violin you're buying is full size unless you're very small.
The violin is a small instrument, but there are specially designed smaller sizes available. Buy full size or 4/4, if you're an adult.Expect to spend a few hundred dollars on a decent beginner's violin.
If you're just starting out with the instrument, there's no need to spend an excessive amount of money on a violin, but like most instruments, the quality of the violin generally rises as the price goes up.