I got this STL file which I gonna build on, when I try to convert it a solid body it says it has to many surfaces. How do I do?
that's an old problem, already discussed here many times. you can try meshlab (free) to reduce the surfaces number or import as graphics. anyway, you won't be able to edit the model because it will be made of thousands of small triangles.
The helmet is symmetrical, so one easy option is to try cutting it in half. In just a moment, you'll have ~50% reduction in triangle count!
Different versions of SolidWorks may import the model as is. I've heard that the 2016 release allows for many more triangles to be imported.
A similiar option is to use the "Scan to 3D" plug-in with SolidWorks Pro. It should allow the import of large mesh files, but as Trinityscsp pointed out, the options after import are limited.
A third option is to convert the mesh to surface data. The software to do this is pretty expensive, and smaller details are often lost.
I converted a half helmet to a surface so you can see if it works for your application
AttachmentsDefault.stp, 89.2 MB