Simple way: Draw the box and then animate masks to “draw” each edge.
Complex way:
1. Draw a single straight line with the bezier tool.
2. Set the anchor point to the starting point (left if it is going to draw to the right).
3. Add keyframes to have the end point over the start point to begin drawing, and the second keyframe where the line stops drawing itself.
4. Duplicate 3 times (for the other lines that will make the box) and rotate, change position, and change start times as necessary.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com