To draw straight lines only:
Use the Pencil tool. Hold down the Shift key while drawing your line. If you want the line a particular length, then click once and type in the length and angle desired.
To draw a shape with connecting straight lines, click at each point a line ends, then edit the line to straighten it by zooming in and editing the points with the Direct Selection Tool (hollow arrow).
If you are going to draw lines and curves, it's easier to dray the major straight lines horizontally or vertically then rotate them 45 degrees.
To draw lines and curves in continuous lines:
1. Use the Pencil tool. Draw your straight line while holding down the Shift key to constrain the angle. Stop drawing where the curve begins.
2. Click where the curve ends; a line will form from the last point to that point.
Continue drawing the straight line, then stop where the curve ends, and so on.
3. Now select the Pen Too/Convert Anchor Point Tool and select the points where the curve starts, then drag one of the handles to curve the line.