Brushes are made in two basic shapes: round and flat. To start out, you will not need many brushes. However, buy the best brush you are able to afford. The most expensive brushes are sable. However, many synthetic blends work quite well and a good bush will last a lifetime if taken care of properly. Taklon is a synthetic brush that I have been pleased with. Be wary of very inexpensive brushes, they tend to lose bristles on your painting and do not hold a point for painting details.
To start with, you can be very successful with only 3 brushes. I recommend a 1-1/2 or 2 inch flat brush to lay down washes, a small round brush for detail (#1 round or liner), and a #5, #6, or #7 round brush. These will get your started - one large, one medium, and one small. Some artists only use one brush. You will find as you progress in your painting that you will have a "favorite" brush.
Above is a photo of the 3 sizes of brushes that I recommend: a 1-1/2 inch flat, a #6 round, and a liner.