Buy the best brush you can afford, not only will they give the best finish, they will last many years if cared for. The preferred varnish brush has long, soft bristles made to keep a sharp chiselled-edge when they are bent. Before using a new brush for the first time, clean it by flicking it open gently to release all loose matter (they always contain some dust or loose hairs). Finally, rinse the brush well in mineral spirits & spin or flick it dry.
Keep a brush dedicated for varnish; undissolved residual paint can contaminate your work. Always clean your brush immediately after use, or, at least, store it in mineral spirts for a shor time until you can. Bend the bristles from side to side to massage the solvent down the heel toward the handle to complete remove all residual varnish that has collected near the base of the bristles. Wash a second & third time until the brush is completely clean. A brush should never be allowed to completely dry. After cleaning, store brushing in white diesel fuel, hanging them carefully to keep bristles straight & to avoid the bristles touching the bottom of the container.