Since you are drawing from an image, you’ll need to change the view of your drawing area so that you can see your lines. Changing your view to X-Ray (View > Face Style > X Ray) will let you see both the floor plan image and your lines.
Once you have the flat image placed in your model, you can use the line and shape tools to draw over the lines on your model. Make sure to keep your lines plumb with the axes so that your walls, windows, etc aren’t at weird angles.
To make your lines a specific length, click once to start the line and then enter the length of the line in inches (i.e., 11’1 -- you don’t need to include the inches symbol). Hit Enter and then you’ll have a line! Move around your outer walls to make a box
Make sure that your lines are connected. When you close a shape it will become slightly shaded.
Some tips when drawing your floorplan:
SketchUp has a box in the lower-right corner that displays measurements as you're drawing lines, using the tape measure tool, or many other tasks. When you are typing in the length for a line, it will appear in this box. If you open SketchUp and this box doesn't show up, you can open it.
Turn your lines into walls. You need to add wall width to make your lines into walls. With the Offset tool (F), click on an edge of a wall and move your cursor towards the center of the image. Type in 5 and hit Enter to make your walls 5” thick.