The Marquee tool selects rectangular portions of the bitmap.
With the bitmap selected, from the Bitmap Edit toolbar:
Select the Marquee tool.
Click and drag the cursor over the bitmap.
After creating one marquee, you can add or subtract areas:
Hold Shift and drag to add additional sections to the selection marquee. A small + appears near the bottom of the cursor to indicate Add.
Hold Ctrl (command in Macintosh) and drag to subtract sections from the selection marquee. A small - appears near the bottom of the cursor to indicate Subtract.
Once drawn, click inside the marquee and drag it to move the marquee to the desired position.
Original bitmap with a marquee
Dragging while holding Shift
Final marquee
Original bitmap with a marquee
Dragging while holding Ctrl (command in Macintosh)