To draw perfectly straight lines (assuming thats what you mean?):
select a tool, whether its pencil, pen, etc. Even the eraser can be used.
Click where you want the start of the line.
Hold shift, and click where you want the end of the line. Depending on the version you use, you should see a line between where you first clicked, and now.
Whilst still holding shift, left click to place the line. Then release shift.
Dont make the same mistake as me when i first tried this - forgot the drawing colour was on white