What tool did you use to draw the lines? Did you use a Vector tool—like the Pen tool or the Vector Path tool? If so, you can modify the properties of the resulting path using the Properties panel (along the bottom of the interface). To "fill in" drawn lines, you may want to try the Fill options in that panel.
If you used a Bitmap tool, like the Pencil or the Brush, then your options are more limited; appearance properties must generally be set in advance of drawing.
If you're looking for a vector tool that can be used to draw freestyle, much like the Brush or Pencil, try the Vector Path tool. It can be found just beneath the Pen tool in the Tools panel.