How to model this shape?

If I understand that shape correctly, then I would trace the 'bottom' shape with a spline, make sure that it is symmetrical, and extrude it in the end.
Spline it!

Make a Spline of that side view, add Poly Select Modifier, save as Editable Poly.

* Extrude outward
* Goto Border, select back end, 'Cap'.
* Cut out the window things on the sides via Edge mode, inserting Vertices on edges (Divide) and then adding Edges (Create) to make the Windows or openings.

Also, you can do one half of that front view, use Bezier Corners for very nice curves, then mirror the Spline for the other side of the front surface, Attach and Weld. Then make it a Polygon, extrude, etc.

Bezier Corners are awesome and enable you to set up their Iteriations and curves quite easily. Look up the corners in the Manual to see how you work them.
Ild just make a box and connect lines across and move vertices to give a smooth corner with a smoothing group, then extrude outwards a bit to make the top, again connect lines to make the smooth corners and add smoothing group to it so it appears perfectly round.

Then you can clear up all the un-used lines you connected so you will be left with a very accurate, detailed but low poly model of whatever that is.