Approaching hard surface models/ connecting pieces together, and critique

I'm a bit confused by the process. I tried to make a scooter to practice, but I ran into a few dificulties.

My process was mostly modelling on the side view, making the vertices create a sillouetue  of the blueprint, and then extruding it. 

How do we approach basemeshes when creating vehicles/hard surface objects? How do we connect it all together?

Also, how should I begin to add details to the object? Should I just subdivide and continue? And also, if there's any hard-surface/vehicle tutorials that help go over these initial subjects and how to master them (blocking out/basemesh, detail, etc) could I have links to some? I would revisit the cgcookie lessons, but I haven't renewed things yet. Thank you!