If you are trying to isolate the brush stroke from the jpeg to use as your "model" brush, use the magic wand tool to select it.
Or, you could use the wand tool to select the background and Select/Inverse.
Note: there are a number of ways to actually mask/isolate a selection.
Than, Edit/Define Brush Preset. But, this probably won't achieve your desired results, as you have a stroke as a model, and not an actual brush pattern.
Probably the most important part for creating realistic looking brush strokes is to have a pressure sensitive pen.
Further, you could do a web search for brush presets, by other artists.