Which brush?

Your choice of brush depends on a number of things but the bottom line is you get what you pay for. The cheaper brushes will do the job for a while but and get ratty fairly quickly depending on the quality of the fibre, the glue and the handle. More expensive brushes can last longer, but only if they are properly looked after. Natural fibre brushes such as Sable brushes are more expensive because of the way they are made and are therefore more expensive to begin with. If you are just starting out, buy within your ability to pay. Start with the less expensive until you feel comfortable with a brush in your hand. Then go up. This way your mistakes like getting the brush clogged with paint, forgetting to clean it etc won´t cost as much to you if you ruin a good brush straight off the bat.