Use DeviceContext->Map(...) with D3D11_MAP_WRITE_DISCARD to get a pointer to an empty the buffer.
Fill the data (with memcpy for example) and the call Unmap. Don't write outside of the buffer! It will cause memory corruption and may even hang your computer.
It doesn't really make sense to use the buffer for one float, but I assume it to be a test. I used DXGI_FORMAT_R32G32B32A32_FLOAT as the buffer format since handling things as float4 makes maybe a bit more sense. You'll need at least 3x4 floats to store a typical matrix, but of course probably you'll want to allocate some megabytes of storage (within reason of course).