Um, if you mean when you try to grab a brush you pick up the background as well, then you simply have to set the right mouse colour to the background colour. In other words: you want to pick up a black squiggle as a brush from a white background. Set your RMB colour as white and then pick it up, and you should be fine.
In MSPaint, you have those 2 very colourful icons when you pick up something, select the bottom one and the background will disappear, but again, you have to have the bg colour set as your secondary colour. hope this helped.