It depends on what program you're using. I use gimp, so I'll help you through that. There are tool settings in the tool box, there's a slider with a label called "opacity". The higher you slide it, the less see-through the object you're drawing. Too cut around the shape so there's not a white box round it, find the magic wand, press ctrl + x. If that won't work, right-click, colours > colour to alpha. Press ok in the window that appears.