To obtain a real FFT i can use Float to Complex block ?
Float to Complex will do the same thing you're seeing now — it just writes a zero imaginary component into the stream.
If you need a signal with an actually one-sided spectrum you can use the Hilbert block, which uses the Hilbert transform to generate a 90° phase shifted quadrature component. But that is just wasted compute cycles unless your next signal processing step actually needs that result. For viewing purposes, just ignore the other side of the spectrum. (It would be nice if the QT GUI Frequency Sink had an option to hide it when given float input, but as far as I know, it doesn't.)