Your off to a good start, but before you go any further, remove the sub-surf and solidify modifiers. Now take a good look at your base mesh and work on cleaning up issues that will make the detail modeling much more difficult.
A. look for and remove any tris or Ngons
B. make sure the quads you end up with are all co-planer ( a fancy way of saying that each face is “flat”
C. try to make the topology of the quads “flow” in such a way that you naturally can place edge loops to define the seams or grooves that go around the helmet - in the screenshot I have quickly done this with the Knife tool. In the process of doing so, more tris and ngons have been created that will have to be cleaned up.
So in essence with this modeling method you loop through a process of…
- Create clean, simple mesh that defines the primary form made from all co-planer quads
- Cut / Extrude /Inset additional details into the base mesh - bigger details first (hard edges / seams / etc.)
- Repair topological issues introduced by the added detail
- Repeat the last two steps until finished
Don’t get wrapped up in using as few quads as possible, focus on using just enough in the right places instead.
Your goal is to get the mesh looking “correct” to you without sub-surf at each level of detail, and then make it smooth for rendering with sub-surf - it’s fine to toggle sub-surf off and on to see where you need to add additional edges to control the surface and edge hardness, but I don’t recommend editing with it on as you will end up with non-planer faces like you currently have.
I reccomend you try to get the basic issues cleaned up and then repost your model