Ringing at mosfet source just before turning on
First things first - you are not driving the MOSFET correctly to achieve reasonable performance. The gate voltage you apply is 5 volts relative to ground and your drain is connected to 12 volts. This means that you can never fully turn on the MOSFET and you'll get really poor efficiency. Try activating the gate with a drive voltage that reaches 15 volts OR better still apply a floating gate-source drive voltage by connecting the negative end of V1 directly to the source.

I understand that the above recommendations make it difficult to achieve in your physical set-up but nobody who knows about these things has said that making an efficient buck converter (or any switching converter) is easy.

As for the ringing, that is due to the parasitic capacitance between drain and source causing a 2nd order damped oscillation (in conjunction with L1) when the MOSFET switches off. Pretty normal for most switching converters.